SEC Info - Ecovyst Inc. - ‘10-K’ for 12/31/23

SEC Info℠

Home

My Interests

Help

Please Sign In

Ecovyst Inc. – ‘10-K’ for 12/31/23 – ‘EX-101.DEF’

On: Thursday, 2/29/24, at 4:02pm ET · For: 12/31/23 · Accession #: 1708035-24-70 · File #: 1-38221

Previous ‘10-K’: ‘10-K’ on 2/28/23 for 12/31/22 · Latest ‘10-K’: This Filing · 18 References:

  As Of               Filer                 Filing    For·On·As Docs:Size

 2/29/24  Ecovyst Inc.                      10-K       12/31/23  167:20M

Annual Report — Form 10-K — SEA’34

Filing Table of Contents

Document/Exhibit                   Description                      Pages   Size 

 1: 10-K        Annual Report                                       HTML   3.70M 
 2: EX-21.1     Subsidiaries List                                   HTML     50K 
 3: EX-23.1     Consent of Expert or Counsel                        HTML     42K 
 4: EX-23.2     Consent of Expert or Counsel                        HTML     41K 
 9: EX-97.1     Clawback Policy re: Recovery of Erroneously         HTML     55K 
                Awarded Compensation                                             
 5: EX-31.1     Certification -- §302 - SOA'02                      HTML     47K 
 6: EX-31.2     Certification -- §302 - SOA'02                      HTML     47K 
 7: EX-32.1     Certification -- §906 - SOA'02                      HTML     44K 
 8: EX-32.2     Certification -- §906 - SOA'02                      HTML     44K 
15: R1          Cover                                               HTML    110K 
16: R2          Audit Information                                   HTML     47K 
17: R3          Consolidated Statements of Income                   HTML    175K 
18: R4          Consolidated Statements of Comprehensive Income     HTML     79K 
                (Loss)                                                           
19: R5          Consolidated Balance Sheets                         HTML    154K 
20: R6          Condensed Consolidated Balance Sheets               HTML     64K 
                (Parenthetical)                                                  
21: R7          Consolidated Statements of Stockholders? Equity     HTML    120K 
22: R8          Consolidated Statements of Cash Flows               HTML    174K 
23: R9          Consolidated Statements of Stockholders? Equity     HTML     44K 
                (Parenthetical)                                                  
24: R10         Background and Basis of Presentation                HTML     49K 
25: R11         Summary of Significant Accounting Policies          HTML    106K 
26: R12         New Accounting Standards                            HTML     61K 
27: R13         Divestitures                                        HTML    110K 
28: R14         Revenue from Contracts with Customers               HTML    105K 
29: R15         Fair Value Measurements                             HTML     72K 
30: R16         Stockholders' Equity                                HTML    149K 
31: R17         Other Operating Expense, Net                        HTML     58K 
32: R18         Inventories, Net                                    HTML     56K 
33: R19         Investments in Affiliated Companies                 HTML     79K 
34: R20         Property, Plant and Equipment                       HTML     55K 
35: R21         Leases                                              HTML    146K 
36: R22         Reportable Segments                                 HTML    131K 
37: R23         Goodwill and Other Intangible Assets                HTML    102K 
38: R24         Accrued Liabilities                                 HTML     56K 
39: R25         Long-term Debt                                      HTML     73K 
40: R26         Other Long-term Liabilities                         HTML     54K 
41: R27         Financial Instruments                               HTML     94K 
42: R28         Income Taxes                                        HTML    149K 
43: R29         Benefit Plans                                       HTML    244K 
44: R30         Stock-Based Compensation                            HTML    166K 
45: R31         Earnings per Share                                  HTML     90K 
46: R32         Commitments and Contingent Liabilities              HTML     50K 
47: R33         Related Party Transactions                          HTML     52K 
48: R34         Supplemental Cash Flow Information                  HTML     57K 
49: R35         Subsequent Events                                   HTML     44K 
50: R36         Schedule I - Condensed Financial Information of     HTML    108K 
                Parent                                                           
51: R37         Pay vs Performance Disclosure                       HTML     54K 
52: R38         Insider Trading Arrangements                        HTML     48K 
53: R39         Summary of Significant Accounting Policies          HTML    189K 
                (Policies)                                                       
54: R40         Summary of Significant Accounting Policies          HTML     59K 
                (Tables)                                                         
55: R41         Divestitures (Tables)                               HTML    100K 
56: R42         Revenue from Contracts with Customers Revenue from  HTML     90K 
                Contracts with Customers (Tables)                                
57: R43         Fair Value Measurements (Tables)                    HTML     65K 
58: R44         Stockholders' Equity (Tables)                       HTML    138K 
59: R45         Other Operating Expense, Net (Tables)               HTML     58K 
60: R46         Inventories (Tables)                                HTML     56K 
61: R47         Investments in Affiliated Companies (Tables)        HTML     74K 
62: R48         Property, Plant and Equipment (Tables)              HTML     54K 
63: R49         Leases (Tables)                                     HTML     98K 
64: R50         Reportable Segments (Tables)                        HTML    133K 
65: R51         Goodwill and Other Intangible Assets (Tables)       HTML    138K 
66: R52         Accrued Liabilities Accrued Liabilities (Tables)    HTML     55K 
67: R53         Long-term Debt (Tables)                             HTML     66K 
68: R54         Other Long-term Liabilities Other Long-term         HTML     54K 
                Liabilities (Tables)                                             
69: R55         Financial Instruments (Tables)                      HTML     97K 
70: R56         Income Taxes (Tables)                               HTML    153K 
71: R57         Benefit Plans (Tables)                              HTML    249K 
72: R58         Stock-Based Compensation - (Tables)                 HTML    149K 
73: R59         Earnings per Share (Tables)                         HTML     91K 
74: R60         Supplemental Cash Flow Information (Tables)         HTML     57K 
75: R61         Schedule I - Condensed Financial Information of     HTML    104K 
                Parent (Tables)                                                  
76: R62         Background and Basis of Presentation (Details)      HTML     53K 
77: R63         Summary of Significant Accounting Policies -        HTML     95K 
                Narratives (Details)                                             
78: R64         Summary of Significant Accounting Policies -        HTML     64K 
                Correction of an Error (Details)                                 
79: R65         Divestitures - Narrative (Details)                  HTML    108K 
80: R66         Divestitures - Reconciliation of Loss (Details)     HTML     97K 
81: R67         Divestitures - Income Information (Details)         HTML     88K 
82: R68         Divestitures - Schedule Of Financing Obligation     HTML     53K 
                Assets And Liabilities (Details)                                 
83: R69         Divestitures - Financing Lease Obligation           HTML     61K 
                (Details)                                                        
84: R70         Revenue from Contracts with Customers - Narrative   HTML     53K 
                (Details)                                                        
85: R71         Revenue from Contracts with Customers -             HTML     77K 
                Disaggregated Revenue (Details)                                  
86: R72         Fair Value Measurements - Assets and Liabilities    HTML     70K 
                Measured at Fair Value on a Recurring Basis                      
                (Details)                                                        
87: R73         Stockholders' Equity - Components of AOCI           HTML     70K 
                (Details)                                                        
88: R74         Stockholders' Equity - Pre-tax and After-tax        HTML     72K 
                Components of Other Comprehensive Income (Loss)                  
                (Details)                                                        
89: R75         Stockholders' Equity - Change by Component          HTML     85K 
                (Details)                                                        
90: R76         Stockholders' Equity - Reclassifications out of     HTML     86K 
                AOCI (Details)                                                   
91: R77         Stockholders' Equity - Treasury Stock Repurchases   HTML     74K 
                (Details)                                                        
92: R78         Stockholders' Equity - Dividends Paid (Details)     HTML     44K 
93: R79         Other Operating Expense, Net (Details)              HTML     54K 
94: R80         Inventories (Details)                               HTML     56K 
95: R81         Inventories, Net - Narratives (Details)             HTML     45K 
96: R82         Investments in Affiliated Companies - Narratives    HTML     71K 
                (Details)                                                        
97: R83         Investments in Affiliated Companies - Ownership     HTML     49K 
                Percentage (Details)                                             
98: R84         Investments in Affiliated Companies - Summarized    HTML     77K 
                Balance Sheet (Details)                                          
99: R85         Investments in Affiliated Companies - Summarized    HTML     78K 
                Income Statement (Details)                                       
100: R86         Investments in Affiliated Companies - Rollforward   HTML     54K  
                (Details)                                                        
101: R87         Property, Plant and Equipment (Details)             HTML     59K  
102: R88         Property, Plant and Equipment - Narratives          HTML     44K  
                (Details)                                                        
103: R89         Leases - Narratives (Details)                       HTML     46K  
104: R90         Leases - Right-of-Use Lease Assets and Lease        HTML     66K  
                Liabilities (Details)                                            
105: R91         Leases - Remaining Average Weighted Average Lease   HTML     52K  
                Term and Discount Rates (Details)                                
106: R92         Leases - Lease Maturity Schedule - ASC 842          HTML     78K  
                (Details)                                                        
107: R93         Leases - Other Lease Information (Details)          HTML     53K  
108: R94         Reportable Segments - Narratives (Details)          HTML     46K  
109: R95         Reportable Segments - Summary Financial             HTML     65K  
                Information by Reportable Segment (Details)                      
110: R96         Reportable Segments - Reconciliation of Net Income  HTML     87K  
                to Adjusted EBITDA (Details)                                     
111: R97         Reportable Segments - Capital Expenditures          HTML     53K  
                (Details)                                                        
112: R98         Reportable Segments - Sales by Geography (Details)  HTML     59K  
113: R99         Reportable Segments - Assets by Geography           HTML     48K  
                (Details)                                                        
114: R100        Goodwill and Other Intangible Assets - Goodwill     HTML     57K  
                Rollforward (Details)                                            
115: R101        Goodwill and Other Intangible Assets - Narratives   HTML     74K  
                (Details)                                                        
116: R102        Goodwill and Other Intangible Assets - Schedule of  HTML     96K  
                Intangible Assets (Details)                                      
117: R103        Goodwill and Other Intangible Assets - Future       HTML     58K  
                Amortization (Details)                                           
118: R104        Accrued Liabilities Accrued Liabilities (Details)   HTML     60K  
119: R105        Long-term Debt (Details)                            HTML     69K  
120: R106        Long-term Debt - ABL Facility (Details)             HTML     93K  
121: R107        Long-term Debt - 2021 Term Loan Facility (Details)  HTML     76K  
122: R108        Long-term Debt - Fair Value (Details)               HTML     52K  
123: R109        Long-term Debt - Aggregate Long-term Debt           HTML     59K  
                Maturities (Details)                                             
124: R110        Other Long-term Liabilities Other Long-term         HTML     58K  
                Liabilities (Details)                                            
125: R111        Financial Instruments - Interest Rate Cap           HTML     66K  
                Agreements (Details)                                             
126: R112        Financial Instruments - Fair Value (Details)        HTML     81K  
127: R113        Financial Instruments - Effect on Other             HTML     52K  
                Comprehensive Income (Details)                                   
128: R114        Financial Instruments - Cash Flow Hedge Impact on   HTML     50K  
                Statements of Income (Details)                                   
129: R115        Income Taxes - Income (loss) before Income Taxes    HTML     52K  
                and Noncontrolling interest (Details)                            
130: R116        Income Taxes - Provision for Income Tax (Details)   HTML     70K  
131: R117        Income Taxes - Reconciliation of Statutory Tax      HTML     69K  
                Rate (Details)                                                   
132: R118        Income Taxes - Components of Deferred Tax Asset     HTML     83K  
                (Liability) (Details)                                            
133: R119        Income Taxes - Narratives (Details)                 HTML     74K  
134: R120        Income Taxes - Schedule of Unrecognized Tax         HTML     47K  
                Benefits Roll Forward (Details)                                  
135: R121        Income Taxes - Payment for Taxes (Details)          HTML     51K  
136: R122        Income Taxes - Inflation Reduction Act (Details)    HTML     47K  
137: R123        Benefit Plans - Change in Benefit Obligation and    HTML     92K  
                Plan Assets (Details)                                            
138: R124        Benefit Plans - Recognized on the Balance Sheet     HTML     58K  
                (Details)                                                        
139: R125        Benefit Plans - Accumulated Other Comprehensive     HTML     59K  
                Income (Details)                                                 
140: R126        Benefit Plans - Net Periodic Benefit Cost           HTML     70K  
                (Details)                                                        
141: R127        Benefit Plans - Amounts recognized in NPBC and OCI  HTML     61K  
                (Details)                                                        
142: R128        Benefit Plans - Projected and Accumulated Benefit   HTML     53K  
                Obligation (Details)                                             
143: R129        Benefit Plans - Weighted Average Assumptions for    HTML     48K  
                Pension Obligation (Details)                                     
144: R130        Benefit Plans - Weighted Average Assumptions for    HTML     57K  
                Net Periodic Cost (Details)                                      
145: R131        Benefit Plans - Plan Assets (Details)               HTML     94K  
146: R132        Benefit Plans - Benefit Plan, Future Payments       HTML     62K  
                (Details)                                                        
147: R133        Benefit Plans - Defined Contribution Plan           HTML     46K  
                (Details)                                                        
148: R134        Stock-Based Compensation - Narratives (Details)     HTML    119K  
149: R135        Stock-Based Compensation - Stock Option Activity    HTML     88K  
                (Details)                                                        
150: R136        Stock-Based Compensation - Restricted Stock         HTML    138K  
                Awards, Restricted Stock Units and Performance                   
                Stock Units (Details)                                            
151: R137        Stock-Based Compensation - Total Stock-Based        HTML     75K  
                Compensation Expense (Details)                                   
152: R138        Reconciliation from Basic to Diluted Weighted       HTML     50K  
                Average Shares Outstanding (Details)                             
153: R139        Calculation of Basic and Diluted Earnings per       HTML    103K  
                Shares (Details)                                                 
154: R140        Anti-dilutive Shares (Details)                      HTML     54K  
155: R141        Commitments and Contingent Liabilities -            HTML     57K  
                Narratives (Details)                                             
156: R142        Related Party Transactions (Details)                HTML     77K  
157: R143        Supplemental Cash Flow Information (Details)        HTML     57K  
158: R144        Schedule I - Condensed Statements of Income         HTML     80K  
                (Details)                                                        
159: R145        Schedule I - Condensed Balance Sheet (Details)      HTML     97K  
160: R146        Schedule I - Condensed Balance Sheet Shares Data    HTML     69K  
                (Details)                                                        
161: R147        Schedule I - Condensed Statement of Cash Flow       HTML     92K  
                (Details)                                                        
162: R148        Schedule I - Narrative (Details)                    HTML     48K  
164: XML         IDEA XML File -- Filing Summary                      XML    311K  
167: XML         XBRL Instance -- ecvt-20231231_htm                   XML   4.72M  
163: EXCEL       IDEA Workbook of Financial Report Info              XLSX    337K  
11: EX-101.CAL  XBRL Calculations -- ecvt-20231231_cal               XML    412K 
12: EX-101.DEF  XBRL Definitions -- ecvt-20231231_def                XML   1.39M 
13: EX-101.LAB  XBRL Labels -- ecvt-20231231_lab                     XML   3.35M 
14: EX-101.PRE  XBRL Presentations -- ecvt-20231231_pre              XML   2.26M 
10: EX-101.SCH  XBRL Schema -- ecvt-20231231                         XSD    330K 
165: JSON        XBRL Instance as JSON Data -- MetaLinks              934±  1.40M  
166: ZIP         XBRL Zipped Folder -- 0001708035-24-000070-xbrl      Zip   1.20M

‘EX-101.DEF’ — XBRL Definitions — ecvt-20231231_def

This Exhibit is an XBRL File.

<?xml version="1.0" encoding="windows-1252"?>

<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">

<link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>

<link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>

<link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>

<link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>

<link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>

<link:roleRef roleURI="http://www.ecovyst.com/role/CONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="simple" xlink:href="ecvt-20231231.xsd#CONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/CONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_165f9e6f-299d-43d9-af53-07c29d9c35bb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_8324b941-d857-422f-9664-64d8642c64fc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_165f9e6f-299d-43d9-af53-07c29d9c35bb" xlink:to="loc_us-gaap_StatementTable_8324b941-d857-422f-9664-64d8642c64fc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_8f1d4860-abc5-45a9-b8ca-18085cccb13a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementEquityComponentsAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_8324b941-d857-422f-9664-64d8642c64fc" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_8f1d4860-abc5-45a9-b8ca-18085cccb13a" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_8f1d4860-abc5-45a9-b8ca-18085cccb13a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_8f1d4860-abc5-45a9-b8ca-18085cccb13a" xlink:to="loc_us-gaap_EquityComponentDomain_8f1d4860-abc5-45a9-b8ca-18085cccb13a_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_703fd15e-f879-461d-b6aa-8d590a225b6a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_8f1d4860-abc5-45a9-b8ca-18085cccb13a" xlink:to="loc_us-gaap_EquityComponentDomain_703fd15e-f879-461d-b6aa-8d590a225b6a" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_b2de1ce1-ed13-4550-b2ac-6cedde4f1c54" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_703fd15e-f879-461d-b6aa-8d590a225b6a" xlink:to="loc_us-gaap_CommonStockMember_b2de1ce1-ed13-4550-b2ac-6cedde4f1c54" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_5b7d97cb-cd93-49de-b38b-883c0471fbfd" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AdditionalPaidInCapitalMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_703fd15e-f879-461d-b6aa-8d590a225b6a" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_5b7d97cb-cd93-49de-b38b-883c0471fbfd" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_b58ad06f-4156-4ef9-ae42-3d3d0e8dd7fb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetainedEarningsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_703fd15e-f879-461d-b6aa-8d590a225b6a" xlink:to="loc_us-gaap_RetainedEarningsMember_b58ad06f-4156-4ef9-ae42-3d3d0e8dd7fb" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonMember_033fad9f-57b1-43f5-959c-66a0677bf9a2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TreasuryStockCommonMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_703fd15e-f879-461d-b6aa-8d590a225b6a" xlink:to="loc_us-gaap_TreasuryStockCommonMember_033fad9f-57b1-43f5-959c-66a0677bf9a2" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_2e34590b-2000-4849-8ad5-24f4c4b6a5fd" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_703fd15e-f879-461d-b6aa-8d590a225b6a" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_2e34590b-2000-4849-8ad5-24f4c4b6a5fd" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncontrollingInterestMember_150d9bbc-45b8-4f90-8738-9944e7911dfd" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NoncontrollingInterestMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_703fd15e-f879-461d-b6aa-8d590a225b6a" xlink:to="loc_us-gaap_NoncontrollingInterestMember_150d9bbc-45b8-4f90-8738-9944e7911dfd" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1c54f8c7-4f58-40b1-9700-c68d7547957f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_165f9e6f-299d-43d9-af53-07c29d9c35bb" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1c54f8c7-4f58-40b1-9700-c68d7547957f" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_f2256f59-47dd-4f60-909e-2608c7dd522d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockSharesOutstanding"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1c54f8c7-4f58-40b1-9700-c68d7547957f" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_f2256f59-47dd-4f60-909e-2608c7dd522d" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_f6d94e08-f636-4328-97d4-26b43e69eab7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1c54f8c7-4f58-40b1-9700-c68d7547957f" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_f6d94e08-f636-4328-97d4-26b43e69eab7" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonShares_4a8eb94d-84bf-4297-9ddd-7af996924fa8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TreasuryStockCommonShares"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1c54f8c7-4f58-40b1-9700-c68d7547957f" xlink:to="loc_us-gaap_TreasuryStockCommonShares_4a8eb94d-84bf-4297-9ddd-7af996924fa8" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_d94a88a3-4be5-4d60-bf82-7573271a8c04" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ProfitLoss"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1c54f8c7-4f58-40b1-9700-c68d7547957f" xlink:to="loc_us-gaap_ProfitLoss_d94a88a3-4be5-4d60-bf82-7573271a8c04" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_7f718f23-e73d-4053-a2e8-48c2df4331a7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1c54f8c7-4f58-40b1-9700-c68d7547957f" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_7f718f23-e73d-4053-a2e8-48c2df4331a7" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockSharesAcquired_f70298fb-a17b-4f6b-b4af-819e2cea15d5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TreasuryStockSharesAcquired"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1c54f8c7-4f58-40b1-9700-c68d7547957f" xlink:to="loc_us-gaap_TreasuryStockSharesAcquired_f70298fb-a17b-4f6b-b4af-819e2cea15d5" xlink:type="arc" order="6"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_fabd020e-ba6a-4ca5-affe-2e3c7e94ede6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TreasuryStockValueAcquiredCostMethod"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1c54f8c7-4f58-40b1-9700-c68d7547957f" xlink:to="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_fabd020e-ba6a-4ca5-affe-2e3c7e94ede6" xlink:type="arc" order="7"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_e306bcef-38dc-42ea-8f55-e319d52481e0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1c54f8c7-4f58-40b1-9700-c68d7547957f" xlink:to="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_e306bcef-38dc-42ea-8f55-e319d52481e0" xlink:type="arc" order="8"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_442dc730-670f-4cf5-9458-8e2be65d2023" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1c54f8c7-4f58-40b1-9700-c68d7547957f" xlink:to="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_442dc730-670f-4cf5-9458-8e2be65d2023" xlink:type="arc" order="9"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_ExciseTaxOnTreasuryStockRepurchased_92ec6f52-ac9f-4d00-ae3e-d7f87905a4c8" xlink:href="ecvt-20231231.xsd#ecvt_ExciseTaxOnTreasuryStockRepurchased"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1c54f8c7-4f58-40b1-9700-c68d7547957f" xlink:to="loc_ecvt_ExciseTaxOnTreasuryStockRepurchased_92ec6f52-ac9f-4d00-ae3e-d7f87905a4c8" xlink:type="arc" order="10"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders_22f798f2-e894-4cda-be86-400cb238f295" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1c54f8c7-4f58-40b1-9700-c68d7547957f" xlink:to="loc_us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders_22f798f2-e894-4cda-be86-400cb238f295" xlink:type="arc" order="11"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsCommonStock_54ed8629-908a-4fc0-adee-55f6e8b0615d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DividendsCommonStock"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1c54f8c7-4f58-40b1-9700-c68d7547957f" xlink:to="loc_us-gaap_DividendsCommonStock_54ed8629-908a-4fc0-adee-55f6e8b0615d" xlink:type="arc" order="12"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_508d3ee4-e127-4f97-be57-1bce78dda30a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1c54f8c7-4f58-40b1-9700-c68d7547957f" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_508d3ee4-e127-4f97-be57-1bce78dda30a" xlink:type="arc" order="13"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_e3903f7b-787a-440d-b3c7-7b3b245e7599" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1c54f8c7-4f58-40b1-9700-c68d7547957f" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_e3903f7b-787a-440d-b3c7-7b3b245e7599" xlink:type="arc" order="14"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_c7943b85-a1dd-4b02-9948-4a4596dc1e9c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_1c54f8c7-4f58-40b1-9700-c68d7547957f" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_c7943b85-a1dd-4b02-9948-4a4596dc1e9c" xlink:type="arc" order="15"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_0caa06b9-02e3-4d13-9271-9e2b028bf9e9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockSharesOutstanding"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_f1ca4b5d-d89e-4c5d-9f0d-34145ff703ad" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonShares_27bd9a19-a5b0-45f8-a3be-e2c6b186d333" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TreasuryStockCommonShares"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/BackgroundandBasisofPresentationDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#BackgroundandBasisofPresentationDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/BackgroundandBasisofPresentationDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_8653ad26-1967-464d-aa75-ed602bc6025c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_4a56a98e-25ce-4d8e-950b-80a4519b8899" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_8653ad26-1967-464d-aa75-ed602bc6025c" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_4a56a98e-25ce-4d8e-950b-80a4519b8899" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_18b2a5e0-fcce-4856-aec3-58e4b98a158d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_4a56a98e-25ce-4d8e-950b-80a4519b8899" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_18b2a5e0-fcce-4856-aec3-58e4b98a158d" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_18b2a5e0-fcce-4856-aec3-58e4b98a158d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_18b2a5e0-fcce-4856-aec3-58e4b98a158d" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_18b2a5e0-fcce-4856-aec3-58e4b98a158d_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_d3935a39-8e7f-4178-a10b-eaf0e3ea56d4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_18b2a5e0-fcce-4856-aec3-58e4b98a158d" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_d3935a39-8e7f-4178-a10b-eaf0e3ea56d4" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PerformanceMaterialsMember_671dc633-0751-4bd3-8c15-cf11fd70295b" xlink:href="ecvt-20231231.xsd#ecvt_PerformanceMaterialsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_d3935a39-8e7f-4178-a10b-eaf0e3ea56d4" xlink:to="loc_ecvt_PerformanceMaterialsMember_671dc633-0751-4bd3-8c15-cf11fd70295b" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PerformanceChemicalsMember_b21623c5-ae80-4a57-af96-3989233e3fec" xlink:href="ecvt-20231231.xsd#ecvt_PerformanceChemicalsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_d3935a39-8e7f-4178-a10b-eaf0e3ea56d4" xlink:to="loc_ecvt_PerformanceChemicalsMember_b21623c5-ae80-4a57-af96-3989233e3fec" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationConsideration_1f93887d-f826-489a-bacb-89cef432889c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationConsideration"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_8653ad26-1967-464d-aa75-ed602bc6025c" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationConsideration_1f93887d-f826-489a-bacb-89cef432889c" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromDivestitureOfBusinesses_c817731b-a80f-4223-966e-31c84e17a870" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ProceedsFromDivestitureOfBusinesses"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_8653ad26-1967-464d-aa75-ed602bc6025c" xlink:to="loc_us-gaap_ProceedsFromDivestitureOfBusinesses_c817731b-a80f-4223-966e-31c84e17a870" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfReportableSegments_92754b43-c128-46c2-9801-c535f80e926b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NumberOfReportableSegments"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_8653ad26-1967-464d-aa75-ed602bc6025c" xlink:to="loc_us-gaap_NumberOfReportableSegments_92754b43-c128-46c2-9801-c535f80e926b" xlink:type="arc" order="2"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/SummaryofSignificantAccountingPoliciesNarrativesDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#SummaryofSignificantAccountingPoliciesNarrativesDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/SummaryofSignificantAccountingPoliciesNarrativesDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_ecvt_SummaryofSignificantAccountingPoliciesTableLineItems_036e6321-644b-484f-bf3f-8ddb5c885c45" xlink:href="ecvt-20231231.xsd#ecvt_SummaryofSignificantAccountingPoliciesTableLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_SummaryofSignificantAccountingPoliciesTable_f3dfe2a1-7202-4053-8970-a2c4236e4f52" xlink:href="ecvt-20231231.xsd#ecvt_SummaryofSignificantAccountingPoliciesTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_ecvt_SummaryofSignificantAccountingPoliciesTableLineItems_036e6321-644b-484f-bf3f-8ddb5c885c45" xlink:to="loc_ecvt_SummaryofSignificantAccountingPoliciesTable_f3dfe2a1-7202-4053-8970-a2c4236e4f52" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_97e50615-5bf4-47c3-92ac-39b2fec215a5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ecvt_SummaryofSignificantAccountingPoliciesTable_f3dfe2a1-7202-4053-8970-a2c4236e4f52" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_97e50615-5bf4-47c3-92ac-39b2fec215a5" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_97e50615-5bf4-47c3-92ac-39b2fec215a5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_97e50615-5bf4-47c3-92ac-39b2fec215a5" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_97e50615-5bf4-47c3-92ac-39b2fec215a5_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_3745ddbc-56eb-4060-bb69-5b057357e7df" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_97e50615-5bf4-47c3-92ac-39b2fec215a5" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_3745ddbc-56eb-4060-bb69-5b057357e7df" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_BuildingAndBuildingImprovementsMember_4cbd8582-f315-4272-bca6-33f9bfa3c12e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BuildingAndBuildingImprovementsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_3745ddbc-56eb-4060-bb69-5b057357e7df" xlink:to="loc_us-gaap_BuildingAndBuildingImprovementsMember_4cbd8582-f315-4272-bca6-33f9bfa3c12e" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_MachineryAndEquipmentMember_f05854e1-0f42-4e6b-a6fc-bdeb0a589919" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MachineryAndEquipmentMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_3745ddbc-56eb-4060-bb69-5b057357e7df" xlink:to="loc_us-gaap_MachineryAndEquipmentMember_f05854e1-0f42-4e6b-a6fc-bdeb0a589919" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_2c7ba27f-cac0-449e-9b71-a6aa6c78ff1c" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ecvt_SummaryofSignificantAccountingPoliciesTable_f3dfe2a1-7202-4053-8970-a2c4236e4f52" xlink:to="loc_srt_RangeAxis_2c7ba27f-cac0-449e-9b71-a6aa6c78ff1c" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_2c7ba27f-cac0-449e-9b71-a6aa6c78ff1c_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_2c7ba27f-cac0-449e-9b71-a6aa6c78ff1c" xlink:to="loc_srt_RangeMember_2c7ba27f-cac0-449e-9b71-a6aa6c78ff1c_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_df14441d-bdf3-495d-b140-8dc8276020b3" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_2c7ba27f-cac0-449e-9b71-a6aa6c78ff1c" xlink:to="loc_srt_RangeMember_df14441d-bdf3-495d-b140-8dc8276020b3" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_09e6c1ec-9ee8-4786-8fa0-dad62a7a6dc6" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MinimumMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_df14441d-bdf3-495d-b140-8dc8276020b3" xlink:to="loc_srt_MinimumMember_09e6c1ec-9ee8-4786-8fa0-dad62a7a6dc6" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_eb08fb31-9156-411c-9cd2-400d30bbf578" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MaximumMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_df14441d-bdf3-495d-b140-8dc8276020b3" xlink:to="loc_srt_MaximumMember_eb08fb31-9156-411c-9cd2-400d30bbf578" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RestatementAxis_f2969b87-9b75-4915-8c61-def4c4026cd6" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RestatementAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ecvt_SummaryofSignificantAccountingPoliciesTable_f3dfe2a1-7202-4053-8970-a2c4236e4f52" xlink:to="loc_srt_RestatementAxis_f2969b87-9b75-4915-8c61-def4c4026cd6" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_f2969b87-9b75-4915-8c61-def4c4026cd6_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RestatementDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RestatementAxis_f2969b87-9b75-4915-8c61-def4c4026cd6" xlink:to="loc_srt_RestatementDomain_f2969b87-9b75-4915-8c61-def4c4026cd6_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_8b016d70-155e-4300-8fc9-dbd040c354ca" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RestatementDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RestatementAxis_f2969b87-9b75-4915-8c61-def4c4026cd6" xlink:to="loc_srt_RestatementDomain_8b016d70-155e-4300-8fc9-dbd040c354ca" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RevisionOfPriorPeriodErrorCorrectionAdjustmentMember_4683f000-898c-4da0-90ff-50ced9a0a30a" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RevisionOfPriorPeriodErrorCorrectionAdjustmentMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_8b016d70-155e-4300-8fc9-dbd040c354ca" xlink:to="loc_srt_RevisionOfPriorPeriodErrorCorrectionAdjustmentMember_4683f000-898c-4da0-90ff-50ced9a0a30a" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCurrencyTransactionGainLossRealized_d59fecef-2e54-4b65-86ec-366150cf1bb2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ForeignCurrencyTransactionGainLossRealized"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecvt_SummaryofSignificantAccountingPoliciesTableLineItems_036e6321-644b-484f-bf3f-8ddb5c885c45" xlink:to="loc_us-gaap_ForeignCurrencyTransactionGainLossRealized_d59fecef-2e54-4b65-86ec-366150cf1bb2" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCash_f73d1908-6f28-4acc-8529-085bcba1df53" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestrictedCash"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecvt_SummaryofSignificantAccountingPoliciesTableLineItems_036e6321-644b-484f-bf3f-8ddb5c885c45" xlink:to="loc_us-gaap_RestrictedCash_f73d1908-6f28-4acc-8529-085bcba1df53" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_425f11bf-06a2-4d13-b11e-c11eedc536c4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecvt_SummaryofSignificantAccountingPoliciesTableLineItems_036e6321-644b-484f-bf3f-8ddb5c885c45" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_425f11bf-06a2-4d13-b11e-c11eedc536c4" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestCostsCapitalized_f95a74ea-b07e-4eda-91cb-c86b34067b4f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_InterestCostsCapitalized"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecvt_SummaryofSignificantAccountingPoliciesTableLineItems_036e6321-644b-484f-bf3f-8ddb5c885c45" xlink:to="loc_us-gaap_InterestCostsCapitalized_f95a74ea-b07e-4eda-91cb-c86b34067b4f" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_LesseeRemainingLeaseTerm_477c1bed-cab0-4454-8b59-e49b9d6b1cd5" xlink:href="ecvt-20231231.xsd#ecvt_LesseeRemainingLeaseTerm"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecvt_SummaryofSignificantAccountingPoliciesTableLineItems_036e6321-644b-484f-bf3f-8ddb5c885c45" xlink:to="loc_ecvt_LesseeRemainingLeaseTerm_477c1bed-cab0-4454-8b59-e49b9d6b1cd5" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_a84ce492-d711-4490-9e56-5ed5862f7cf1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ResearchAndDevelopmentExpense"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecvt_SummaryofSignificantAccountingPoliciesTableLineItems_036e6321-644b-484f-bf3f-8ddb5c885c45" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_a84ce492-d711-4490-9e56-5ed5862f7cf1" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_a4094de4-359d-477f-b830-093cfa4ad43d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_InterestExpense"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecvt_SummaryofSignificantAccountingPoliciesTableLineItems_036e6321-644b-484f-bf3f-8ddb5c885c45" xlink:to="loc_us-gaap_InterestExpense_a4094de4-359d-477f-b830-093cfa4ad43d" xlink:type="arc" order="6"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_SalesRebateReserve_27817573-932f-4187-9f7d-d2f124d97da8" xlink:href="ecvt-20231231.xsd#ecvt_SalesRebateReserve"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecvt_SummaryofSignificantAccountingPoliciesTableLineItems_036e6321-644b-484f-bf3f-8ddb5c885c45" xlink:to="loc_ecvt_SalesRebateReserve_27817573-932f-4187-9f7d-d2f124d97da8" xlink:type="arc" order="7"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestments_14dffe8e-156e-41c2-8562-f4aaf1ae0923" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeLossFromEquityMethodInvestments"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecvt_SummaryofSignificantAccountingPoliciesTableLineItems_036e6321-644b-484f-bf3f-8ddb5c885c45" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestments_14dffe8e-156e-41c2-8562-f4aaf1ae0923" xlink:type="arc" order="8"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherExpenses_bc830001-5a59-41e7-942d-a96f9955b9a1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherExpenses"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecvt_SummaryofSignificantAccountingPoliciesTableLineItems_036e6321-644b-484f-bf3f-8ddb5c885c45" xlink:to="loc_us-gaap_OtherExpenses_bc830001-5a59-41e7-942d-a96f9955b9a1" xlink:type="arc" order="9"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_246cc113-d624-431d-9b33-4f87347ca51c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NetIncomeLoss"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecvt_SummaryofSignificantAccountingPoliciesTableLineItems_036e6321-644b-484f-bf3f-8ddb5c885c45" xlink:to="loc_us-gaap_NetIncomeLoss_246cc113-d624-431d-9b33-4f87347ca51c" xlink:type="arc" order="10"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_NumberOfDefinedBenefitPensionPlans_bdbdafec-610e-423d-a4d2-d4ab2ee30e20" xlink:href="ecvt-20231231.xsd#ecvt_NumberOfDefinedBenefitPensionPlans"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecvt_SummaryofSignificantAccountingPoliciesTableLineItems_036e6321-644b-484f-bf3f-8ddb5c885c45" xlink:to="loc_ecvt_NumberOfDefinedBenefitPensionPlans_bdbdafec-610e-423d-a4d2-d4ab2ee30e20" xlink:type="arc" order="11"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/SummaryofSignificantAccountingPoliciesCorrectionofanErrorDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#SummaryofSignificantAccountingPoliciesCorrectionofanErrorDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/SummaryofSignificantAccountingPoliciesCorrectionofanErrorDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_ecvt_SummaryOfSignificantAccountingPoliciesLineItems_b1d78565-cedd-468a-bfe8-aaa3c8bf117f" xlink:href="ecvt-20231231.xsd#ecvt_SummaryOfSignificantAccountingPoliciesLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_SummaryofSignificantAccountingPoliciesTable_5b5ca7c0-9277-4970-a46a-b5611c3943b9" xlink:href="ecvt-20231231.xsd#ecvt_SummaryofSignificantAccountingPoliciesTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_ecvt_SummaryOfSignificantAccountingPoliciesLineItems_b1d78565-cedd-468a-bfe8-aaa3c8bf117f" xlink:to="loc_ecvt_SummaryofSignificantAccountingPoliciesTable_5b5ca7c0-9277-4970-a46a-b5611c3943b9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RestatementAxis_716fe671-ed64-4056-bba3-2019e06e49fa" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RestatementAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ecvt_SummaryofSignificantAccountingPoliciesTable_5b5ca7c0-9277-4970-a46a-b5611c3943b9" xlink:to="loc_srt_RestatementAxis_716fe671-ed64-4056-bba3-2019e06e49fa" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_716fe671-ed64-4056-bba3-2019e06e49fa_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RestatementDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RestatementAxis_716fe671-ed64-4056-bba3-2019e06e49fa" xlink:to="loc_srt_RestatementDomain_716fe671-ed64-4056-bba3-2019e06e49fa_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_f10fc091-9720-4a57-82a7-661d0b108ce5" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RestatementDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RestatementAxis_716fe671-ed64-4056-bba3-2019e06e49fa" xlink:to="loc_srt_RestatementDomain_f10fc091-9720-4a57-82a7-661d0b108ce5" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioPreviouslyReportedMember_dfd76b07-ec57-4af3-aa88-e8f47503a18b" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ScenarioPreviouslyReportedMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_f10fc091-9720-4a57-82a7-661d0b108ce5" xlink:to="loc_srt_ScenarioPreviouslyReportedMember_dfd76b07-ec57-4af3-aa88-e8f47503a18b" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RevisionOfPriorPeriodErrorCorrectionAdjustmentMember_0bc41656-cd0d-46ed-aa29-cf5fb05571c7" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RevisionOfPriorPeriodErrorCorrectionAdjustmentMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_f10fc091-9720-4a57-82a7-661d0b108ce5" xlink:to="loc_srt_RevisionOfPriorPeriodErrorCorrectionAdjustmentMember_0bc41656-cd0d-46ed-aa29-cf5fb05571c7" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_a121aa5e-110d-457d-9b64-3d7e638abab5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementEquityComponentsAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ecvt_SummaryofSignificantAccountingPoliciesTable_5b5ca7c0-9277-4970-a46a-b5611c3943b9" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_a121aa5e-110d-457d-9b64-3d7e638abab5" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_a121aa5e-110d-457d-9b64-3d7e638abab5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_a121aa5e-110d-457d-9b64-3d7e638abab5" xlink:to="loc_us-gaap_EquityComponentDomain_a121aa5e-110d-457d-9b64-3d7e638abab5_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_2dfff56e-1ae0-4663-95fa-92eab50d2ce6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_a121aa5e-110d-457d-9b64-3d7e638abab5" xlink:to="loc_us-gaap_EquityComponentDomain_2dfff56e-1ae0-4663-95fa-92eab50d2ce6" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_8d85cb57-bed7-4f9d-998c-149ba2287a02" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_2dfff56e-1ae0-4663-95fa-92eab50d2ce6" xlink:to="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_8d85cb57-bed7-4f9d-998c-149ba2287a02" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_2946d909-badf-41df-9d70-07e7203f9417" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_2dfff56e-1ae0-4663-95fa-92eab50d2ce6" xlink:to="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_2946d909-badf-41df-9d70-07e7203f9417" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedTranslationAdjustmentMember_027e5d3e-ea9a-45c2-8193-42dd16ba5dd4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedTranslationAdjustmentMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_2dfff56e-1ae0-4663-95fa-92eab50d2ce6" xlink:to="loc_us-gaap_AccumulatedTranslationAdjustmentMember_027e5d3e-ea9a-45c2-8193-42dd16ba5dd4" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_e5390bc7-0f80-4752-b9cb-74af4b4383b4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockholdersEquity"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecvt_SummaryOfSignificantAccountingPoliciesLineItems_b1d78565-cedd-468a-bfe8-aaa3c8bf117f" xlink:to="loc_us-gaap_StockholdersEquity_e5390bc7-0f80-4752-b9cb-74af4b4383b4" xlink:type="arc" order="0"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/DivestituresNarrativeDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#DivestituresNarrativeDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/DivestituresNarrativeDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_a41e9f6d-3c55-4e2b-9a86-93b97c01d98e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_f3881fd0-f0b2-476e-bf64-516892b411d0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_a41e9f6d-3c55-4e2b-9a86-93b97c01d98e" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_f3881fd0-f0b2-476e-bf64-516892b411d0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_3151e66c-42da-46e3-bc0a-8a124650bcc9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_f3881fd0-f0b2-476e-bf64-516892b411d0" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_3151e66c-42da-46e3-bc0a-8a124650bcc9" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_3151e66c-42da-46e3-bc0a-8a124650bcc9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_3151e66c-42da-46e3-bc0a-8a124650bcc9" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_3151e66c-42da-46e3-bc0a-8a124650bcc9_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_09360d66-fcab-4719-b9b1-afc0bc77bfe0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_3151e66c-42da-46e3-bc0a-8a124650bcc9" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_09360d66-fcab-4719-b9b1-afc0bc77bfe0" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PerformanceChemicalsMember_ec2750d4-2d3e-44ed-a2bc-57d77f352d5d" xlink:href="ecvt-20231231.xsd#ecvt_PerformanceChemicalsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_09360d66-fcab-4719-b9b1-afc0bc77bfe0" xlink:to="loc_ecvt_PerformanceChemicalsMember_ec2750d4-2d3e-44ed-a2bc-57d77f352d5d" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PerformanceMaterialsMember_316df97d-5faa-4081-b771-cc0e69cf0d03" xlink:href="ecvt-20231231.xsd#ecvt_PerformanceMaterialsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_09360d66-fcab-4719-b9b1-afc0bc77bfe0" xlink:to="loc_ecvt_PerformanceMaterialsMember_316df97d-5faa-4081-b771-cc0e69cf0d03" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_DisposalGroupIncludingDiscontinuedOperationPricePerDefinitiveAgreement_3c8edf09-48dd-4d0b-b63f-7dfa22df50de" xlink:href="ecvt-20231231.xsd#ecvt_DisposalGroupIncludingDiscontinuedOperationPricePerDefinitiveAgreement"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_a41e9f6d-3c55-4e2b-9a86-93b97c01d98e" xlink:to="loc_ecvt_DisposalGroupIncludingDiscontinuedOperationPricePerDefinitiveAgreement_3c8edf09-48dd-4d0b-b63f-7dfa22df50de" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested_dfdc4fc6-f80f-444f-bf3a-c4ab2fadfc5a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_a41e9f6d-3c55-4e2b-9a86-93b97c01d98e" xlink:to="loc_us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested_dfdc4fc6-f80f-444f-bf3a-c4ab2fadfc5a" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockDividendsPerShareCashPaid_796a9000-b698-4d3f-aba3-e35a5d8a3233" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockDividendsPerShareCashPaid"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_a41e9f6d-3c55-4e2b-9a86-93b97c01d98e" xlink:to="loc_us-gaap_CommonStockDividendsPerShareCashPaid_796a9000-b698-4d3f-aba3-e35a5d8a3233" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationConsideration_ae16258d-13d6-444f-ab17-206ed31dc5bb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationConsideration"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_a41e9f6d-3c55-4e2b-9a86-93b97c01d98e" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationConsideration_ae16258d-13d6-444f-ab17-206ed31dc5bb" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_DisposalGroupIncludingDiscontinuedOperationTransactionCostsForDisposal_7a11759e-2066-4461-97ef-6fbdeb2e03e4" xlink:href="ecvt-20231231.xsd#ecvt_DisposalGroupIncludingDiscontinuedOperationTransactionCostsForDisposal"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_a41e9f6d-3c55-4e2b-9a86-93b97c01d98e" xlink:to="loc_ecvt_DisposalGroupIncludingDiscontinuedOperationTransactionCostsForDisposal_7a11759e-2066-4461-97ef-6fbdeb2e03e4" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_c709d749-a6d9-46b0-9dd2-8136464cbe50" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_a41e9f6d-3c55-4e2b-9a86-93b97c01d98e" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_c709d749-a6d9-46b0-9dd2-8136464cbe50" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_IncomeTaxBenefitOnTransactionCostsAndStockBasedCompensationExpenseOnly_2925c79e-c868-4d97-bca0-43b7195f79ec" xlink:href="ecvt-20231231.xsd#ecvt_IncomeTaxBenefitOnTransactionCostsAndStockBasedCompensationExpenseOnly"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_a41e9f6d-3c55-4e2b-9a86-93b97c01d98e" xlink:to="loc_ecvt_IncomeTaxBenefitOnTransactionCostsAndStockBasedCompensationExpenseOnly_2925c79e-c868-4d97-bca0-43b7195f79ec" xlink:type="arc" order="6"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_ProvisionToReturnAdjustmentRecognized_9e74138d-184f-423a-94c3-5f6c40cffe42" xlink:href="ecvt-20231231.xsd#ecvt_ProvisionToReturnAdjustmentRecognized"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_a41e9f6d-3c55-4e2b-9a86-93b97c01d98e" xlink:to="loc_ecvt_ProvisionToReturnAdjustmentRecognized_9e74138d-184f-423a-94c3-5f6c40cffe42" xlink:type="arc" order="7"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_TransitionServicesAgreementTerm_8111200e-3f55-4662-8a99-8598205c0b6f" xlink:href="ecvt-20231231.xsd#ecvt_TransitionServicesAgreementTerm"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_a41e9f6d-3c55-4e2b-9a86-93b97c01d98e" xlink:to="loc_ecvt_TransitionServicesAgreementTerm_8111200e-3f55-4662-8a99-8598205c0b6f" xlink:type="arc" order="8"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_TransitionServiceAgreementIncome_889e5b08-8978-450c-a48c-374c529aa605" xlink:href="ecvt-20231231.xsd#ecvt_TransitionServiceAgreementIncome"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_a41e9f6d-3c55-4e2b-9a86-93b97c01d98e" xlink:to="loc_ecvt_TransitionServiceAgreementIncome_889e5b08-8978-450c-a48c-374c529aa605" xlink:type="arc" order="9"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_EstimateDisposalLoss_623b1677-f6a5-428a-aa7c-5d34317452b5" xlink:href="ecvt-20231231.xsd#ecvt_EstimateDisposalLoss"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_a41e9f6d-3c55-4e2b-9a86-93b97c01d98e" xlink:to="loc_ecvt_EstimateDisposalLoss_623b1677-f6a5-428a-aa7c-5d34317452b5" xlink:type="arc" order="10"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_FinancingObligationLiability_7e698f5e-a3d3-4d4f-bfcf-b4b4f5a002ce" xlink:href="ecvt-20231231.xsd#ecvt_FinancingObligationLiability"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_a41e9f6d-3c55-4e2b-9a86-93b97c01d98e" xlink:to="loc_ecvt_FinancingObligationLiability_7e698f5e-a3d3-4d4f-bfcf-b4b4f5a002ce" xlink:type="arc" order="11"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_ProceedsFromFailedSaleLeaseback_d4c2af28-8de2-481b-80fe-cec4e6827d14" xlink:href="ecvt-20231231.xsd#ecvt_ProceedsFromFailedSaleLeaseback"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_a41e9f6d-3c55-4e2b-9a86-93b97c01d98e" xlink:to="loc_ecvt_ProceedsFromFailedSaleLeaseback_d4c2af28-8de2-481b-80fe-cec4e6827d14" xlink:type="arc" order="12"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PaymentsUnderFinancingArrangement_e6ca2cc1-e946-48b8-9ac6-354c9fb6da78" xlink:href="ecvt-20231231.xsd#ecvt_PaymentsUnderFinancingArrangement"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_a41e9f6d-3c55-4e2b-9a86-93b97c01d98e" xlink:to="loc_ecvt_PaymentsUnderFinancingArrangement_e6ca2cc1-e946-48b8-9ac6-354c9fb6da78" xlink:type="arc" order="13"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_RepaymentOfFinancingObligation_f1847d51-a046-4170-8f9f-5d88a4599118" xlink:href="ecvt-20231231.xsd#ecvt_RepaymentOfFinancingObligation"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_a41e9f6d-3c55-4e2b-9a86-93b97c01d98e" xlink:to="loc_ecvt_RepaymentOfFinancingObligation_f1847d51-a046-4170-8f9f-5d88a4599118" xlink:type="arc" order="14"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_RepaymentOfInterestOnFinancingObligation_b3a11ec0-1af0-4752-9bb9-d830765bc33a" xlink:href="ecvt-20231231.xsd#ecvt_RepaymentOfInterestOnFinancingObligation"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_a41e9f6d-3c55-4e2b-9a86-93b97c01d98e" xlink:to="loc_ecvt_RepaymentOfInterestOnFinancingObligation_b3a11ec0-1af0-4752-9bb9-d830765bc33a" xlink:type="arc" order="15"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseWeightedAverageRemainingLeaseTerm1_d7db1856-e5d4-44eb-bcf8-4cb0a63be9c8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FinanceLeaseWeightedAverageRemainingLeaseTerm1"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_a41e9f6d-3c55-4e2b-9a86-93b97c01d98e" xlink:to="loc_us-gaap_FinanceLeaseWeightedAverageRemainingLeaseTerm1_d7db1856-e5d4-44eb-bcf8-4cb0a63be9c8" xlink:type="arc" order="16"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseWeightedAverageDiscountRatePercent_5d619ddd-b4a4-4865-8716-80c12417fc0e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FinanceLeaseWeightedAverageDiscountRatePercent"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_a41e9f6d-3c55-4e2b-9a86-93b97c01d98e" xlink:to="loc_us-gaap_FinanceLeaseWeightedAverageDiscountRatePercent_5d619ddd-b4a4-4865-8716-80c12417fc0e" xlink:type="arc" order="17"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToNoncontrollingInterest_06d032ce-79cc-4d8e-996b-8dd9b5e30f56" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToNoncontrollingInterest"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_a41e9f6d-3c55-4e2b-9a86-93b97c01d98e" xlink:to="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToNoncontrollingInterest_06d032ce-79cc-4d8e-996b-8dd9b5e30f56" xlink:type="arc" order="18"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_c71be3ab-9835-45ba-bf84-a0ae6b10a875" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NetIncomeLoss"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_a41e9f6d-3c55-4e2b-9a86-93b97c01d98e" xlink:to="loc_us-gaap_NetIncomeLoss_c71be3ab-9835-45ba-bf84-a0ae6b10a875" xlink:type="arc" order="19"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/DivestituresReconciliationofLossDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#DivestituresReconciliationofLossDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/DivestituresReconciliationofLossDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_8f3679c0-0683-422b-b3a9-4928c30c14d0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_d753f76d-9126-4908-acf8-75421f1485b1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_8f3679c0-0683-422b-b3a9-4928c30c14d0" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_d753f76d-9126-4908-acf8-75421f1485b1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_6e44005e-b18b-477f-827f-088c30f20940" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_d753f76d-9126-4908-acf8-75421f1485b1" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_6e44005e-b18b-477f-827f-088c30f20940" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_6e44005e-b18b-477f-827f-088c30f20940_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_6e44005e-b18b-477f-827f-088c30f20940" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_6e44005e-b18b-477f-827f-088c30f20940_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_7e164179-34bc-459b-b509-6c200fd72d07" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_6e44005e-b18b-477f-827f-088c30f20940" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_7e164179-34bc-459b-b509-6c200fd72d07" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PerformanceChemicalsMember_94f524fd-80d5-49e0-9d5e-e5758951bdbc" xlink:href="ecvt-20231231.xsd#ecvt_PerformanceChemicalsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_7e164179-34bc-459b-b509-6c200fd72d07" xlink:to="loc_ecvt_PerformanceChemicalsMember_94f524fd-80d5-49e0-9d5e-e5758951bdbc" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_EstimateDisposalLoss_b55f2827-e5bc-4585-b99b-8184d9b36bca" xlink:href="ecvt-20231231.xsd#ecvt_EstimateDisposalLoss"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_8f3679c0-0683-422b-b3a9-4928c30c14d0" xlink:to="loc_ecvt_EstimateDisposalLoss_b55f2827-e5bc-4585-b99b-8184d9b36bca" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested_19f3ca23-400e-40db-afb9-bf9d5512ae60" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_8f3679c0-0683-422b-b3a9-4928c30c14d0" xlink:to="loc_us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested_19f3ca23-400e-40db-afb9-bf9d5512ae60" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_DisposalGroupIncludingDiscontinuedOperationTransactionCostsForDisposal_f622c52a-049a-4bfb-a451-8b0e927c902a" xlink:href="ecvt-20231231.xsd#ecvt_DisposalGroupIncludingDiscontinuedOperationTransactionCostsForDisposal"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_8f3679c0-0683-422b-b3a9-4928c30c14d0" xlink:to="loc_ecvt_DisposalGroupIncludingDiscontinuedOperationTransactionCostsForDisposal_f622c52a-049a-4bfb-a451-8b0e927c902a" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_DisposalGroupIncludingDiscontinuedOperationNetAssetsDerecognizedOnDisposal_e5bd64eb-7a40-40a0-bd96-b1e1dedd4d43" xlink:href="ecvt-20231231.xsd#ecvt_DisposalGroupIncludingDiscontinuedOperationNetAssetsDerecognizedOnDisposal"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_8f3679c0-0683-422b-b3a9-4928c30c14d0" xlink:to="loc_ecvt_DisposalGroupIncludingDiscontinuedOperationNetAssetsDerecognizedOnDisposal_e5bd64eb-7a40-40a0-bd96-b1e1dedd4d43" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax_dd945415-4b63-411d-a0a5-b4106508b72f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_8f3679c0-0683-422b-b3a9-4928c30c14d0" xlink:to="loc_us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax_dd945415-4b63-411d-a0a5-b4106508b72f" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_ef359224-c17a-47e7-ba20-cd50082e5e0c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_8f3679c0-0683-422b-b3a9-4928c30c14d0" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_ef359224-c17a-47e7-ba20-cd50082e5e0c" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DiscontinuedOperationTaxEffectOfDiscontinuedOperation_93d83571-0a2f-4242-8e9d-8071c5924ef5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DiscontinuedOperationTaxEffectOfDiscontinuedOperation"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_8f3679c0-0683-422b-b3a9-4928c30c14d0" xlink:to="loc_us-gaap_DiscontinuedOperationTaxEffectOfDiscontinuedOperation_93d83571-0a2f-4242-8e9d-8071c5924ef5" xlink:type="arc" order="6"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PaymentsForBusinessDivestiture_d564cd8e-3238-4fdf-a07a-6b053971334e" xlink:href="ecvt-20231231.xsd#ecvt_PaymentsForBusinessDivestiture"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_8f3679c0-0683-422b-b3a9-4928c30c14d0" xlink:to="loc_ecvt_PaymentsForBusinessDivestiture_d564cd8e-3238-4fdf-a07a-6b053971334e" xlink:type="arc" order="7"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/DivestituresIncomeInformationDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#DivestituresIncomeInformationDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/DivestituresIncomeInformationDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ad2607dc-ba28-47c6-b3ae-aae7b8da48e9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_5f26f384-7d08-4e57-9d0e-7ab2cd0ff506" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ad2607dc-ba28-47c6-b3ae-aae7b8da48e9" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_5f26f384-7d08-4e57-9d0e-7ab2cd0ff506" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_445e9c28-9df4-4358-a727-bbc510c9ebe4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_5f26f384-7d08-4e57-9d0e-7ab2cd0ff506" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_445e9c28-9df4-4358-a727-bbc510c9ebe4" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_445e9c28-9df4-4358-a727-bbc510c9ebe4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_445e9c28-9df4-4358-a727-bbc510c9ebe4" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_445e9c28-9df4-4358-a727-bbc510c9ebe4_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_ed64104e-4d34-4ca0-ba68-2ea6292e7a43" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_445e9c28-9df4-4358-a727-bbc510c9ebe4" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_ed64104e-4d34-4ca0-ba68-2ea6292e7a43" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PerformanceChemicalsMember_b53cb38c-0c6e-44cd-af40-8fbcbac5fcd6" xlink:href="ecvt-20231231.xsd#ecvt_PerformanceChemicalsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_ed64104e-4d34-4ca0-ba68-2ea6292e7a43" xlink:to="loc_ecvt_PerformanceChemicalsMember_b53cb38c-0c6e-44cd-af40-8fbcbac5fcd6" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PerformanceChemicalsMember_565b25df-62b7-41b4-8ad5-5d055b6a2123" xlink:href="ecvt-20231231.xsd#ecvt_PerformanceChemicalsMember"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue_f9863b73-3903-4218-af14-bd66c8fff198" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ad2607dc-ba28-47c6-b3ae-aae7b8da48e9" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue_f9863b73-3903-4218-af14-bd66c8fff198" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationCostsOfGoodsSold_1a247082-43cb-475d-adea-35451d25e484" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationCostsOfGoodsSold"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ad2607dc-ba28-47c6-b3ae-aae7b8da48e9" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationCostsOfGoodsSold_1a247082-43cb-475d-adea-35451d25e484" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationGeneralAndAdministrativeExpense_959660c9-b798-4084-b8c4-394fad7a6aa4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationGeneralAndAdministrativeExpense"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ad2607dc-ba28-47c6-b3ae-aae7b8da48e9" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationGeneralAndAdministrativeExpense_959660c9-b798-4084-b8c4-394fad7a6aa4" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillImpairmentLoss_526580e9-f5bc-4caf-95f1-051bf8f157c9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_GoodwillImpairmentLoss"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ad2607dc-ba28-47c6-b3ae-aae7b8da48e9" xlink:to="loc_us-gaap_GoodwillImpairmentLoss_526580e9-f5bc-4caf-95f1-051bf8f157c9" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingExpense_5641e938-8bfe-47de-b1cd-04d581cfa3d4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingExpense"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ad2607dc-ba28-47c6-b3ae-aae7b8da48e9" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingExpense_5641e938-8bfe-47de-b1cd-04d581cfa3d4" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax_e6eb4235-65ee-4c34-9a97-084a40eff804" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ad2607dc-ba28-47c6-b3ae-aae7b8da48e9" xlink:to="loc_us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax_e6eb4235-65ee-4c34-9a97-084a40eff804" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingIncomeLoss_2b53a237-c9dc-4830-a1d0-557539bee13b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingIncomeLoss"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ad2607dc-ba28-47c6-b3ae-aae7b8da48e9" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingIncomeLoss_2b53a237-c9dc-4830-a1d0-557539bee13b" xlink:type="arc" order="6"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestments_4c440dbd-f3d2-4c3d-b342-defcd82d5279" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeLossFromEquityMethodInvestments"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ad2607dc-ba28-47c6-b3ae-aae7b8da48e9" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestments_4c440dbd-f3d2-4c3d-b342-defcd82d5279" xlink:type="arc" order="7"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationInterestExpense_fb2d70b9-37a9-4a04-b757-6b1af79b6c3b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationInterestExpense"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ad2607dc-ba28-47c6-b3ae-aae7b8da48e9" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationInterestExpense_fb2d70b9-37a9-4a04-b757-6b1af79b6c3b" xlink:type="arc" order="8"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationOtherIncome_3696ea23-ccc8-46d0-aabb-bfcc43cbf581" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationOtherIncome"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ad2607dc-ba28-47c6-b3ae-aae7b8da48e9" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationOtherIncome_3696ea23-ccc8-46d0-aabb-bfcc43cbf581" xlink:type="arc" order="9"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationBeforeIncomeTax_5960ba12-dafc-4c1f-ae25-c6cc478c8937" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationBeforeIncomeTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ad2607dc-ba28-47c6-b3ae-aae7b8da48e9" xlink:to="loc_us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationBeforeIncomeTax_5960ba12-dafc-4c1f-ae25-c6cc478c8937" xlink:type="arc" order="10"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DiscontinuedOperationTaxEffectOfDiscontinuedOperation_49c79dbe-4d04-4121-b2ab-b90ad08f78d2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DiscontinuedOperationTaxEffectOfDiscontinuedOperation"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ad2607dc-ba28-47c6-b3ae-aae7b8da48e9" xlink:to="loc_us-gaap_DiscontinuedOperationTaxEffectOfDiscontinuedOperation_49c79dbe-4d04-4121-b2ab-b90ad08f78d2" xlink:type="arc" order="11"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax_dbd39cb8-c00f-43a6-9086-945a334ef873" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ad2607dc-ba28-47c6-b3ae-aae7b8da48e9" xlink:to="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax_dbd39cb8-c00f-43a6-9086-945a334ef873" xlink:type="arc" order="12"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfMediumTermNotes_ff088231-2af7-43bf-8222-d209cd293678" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RepaymentsOfMediumTermNotes"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_ad2607dc-ba28-47c6-b3ae-aae7b8da48e9" xlink:to="loc_us-gaap_RepaymentsOfMediumTermNotes_ff088231-2af7-43bf-8222-d209cd293678" xlink:type="arc" order="13"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/DivestituresScheduleOfFinancingObligationAssetsAndLiabilitiesDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#DivestituresScheduleOfFinancingObligationAssetsAndLiabilitiesDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/DivestituresScheduleOfFinancingObligationAssetsAndLiabilitiesDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_ecvt_FinancingObligationsAssetsAndLiabilitiesLineItems_f4c73c31-b728-475b-91a2-9dc6b22d9635" xlink:href="ecvt-20231231.xsd#ecvt_FinancingObligationsAssetsAndLiabilitiesLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_FinancingObligationsAssetsAndLiabilitiesTable_fee73793-5b08-43d3-a9b4-31e8c26ea697" xlink:href="ecvt-20231231.xsd#ecvt_FinancingObligationsAssetsAndLiabilitiesTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_ecvt_FinancingObligationsAssetsAndLiabilitiesLineItems_f4c73c31-b728-475b-91a2-9dc6b22d9635" xlink:to="loc_ecvt_FinancingObligationsAssetsAndLiabilitiesTable_fee73793-5b08-43d3-a9b4-31e8c26ea697" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_929d6856-4247-4849-82e3-d112991547bd" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BalanceSheetLocationAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ecvt_FinancingObligationsAssetsAndLiabilitiesTable_fee73793-5b08-43d3-a9b4-31e8c26ea697" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_929d6856-4247-4849-82e3-d112991547bd" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_929d6856-4247-4849-82e3-d112991547bd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BalanceSheetLocationDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_929d6856-4247-4849-82e3-d112991547bd" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_929d6856-4247-4849-82e3-d112991547bd_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_bb0ba031-3ad5-4874-ab81-f31e658d6f98" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BalanceSheetLocationDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_929d6856-4247-4849-82e3-d112991547bd" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_bb0ba031-3ad5-4874-ab81-f31e658d6f98" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentMember_aba6f859-305f-468c-b2f7-eaf97f543978" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_bb0ba031-3ad5-4874-ab81-f31e658d6f98" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentMember_aba6f859-305f-468c-b2f7-eaf97f543978" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableAndAccruedLiabilitiesMember_ff6c67c1-1f75-45a3-bbb4-20a19765eb33" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccountsPayableAndAccruedLiabilitiesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_bb0ba031-3ad5-4874-ab81-f31e658d6f98" xlink:to="loc_us-gaap_AccountsPayableAndAccruedLiabilitiesMember_ff6c67c1-1f75-45a3-bbb4-20a19765eb33" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNoncurrentLiabilitiesMember_83cf0392-89ff-4880-909b-5ef1f0eb6a5b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherNoncurrentLiabilitiesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_bb0ba031-3ad5-4874-ab81-f31e658d6f98" xlink:to="loc_us-gaap_OtherNoncurrentLiabilitiesMember_83cf0392-89ff-4880-909b-5ef1f0eb6a5b" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseRightOfUseAssetBeforeAccumulatedAmortization_049f33e6-1b89-426d-a0e0-3dfc30d4f051" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FinanceLeaseRightOfUseAssetBeforeAccumulatedAmortization"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecvt_FinancingObligationsAssetsAndLiabilitiesLineItems_f4c73c31-b728-475b-91a2-9dc6b22d9635" xlink:to="loc_us-gaap_FinanceLeaseRightOfUseAssetBeforeAccumulatedAmortization_049f33e6-1b89-426d-a0e0-3dfc30d4f051" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseLiability_96e19dd6-2528-4f7a-830e-122fa83fad7d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FinanceLeaseLiability"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecvt_FinancingObligationsAssetsAndLiabilitiesLineItems_f4c73c31-b728-475b-91a2-9dc6b22d9635" xlink:to="loc_us-gaap_FinanceLeaseLiability_96e19dd6-2528-4f7a-830e-122fa83fad7d" xlink:type="arc" order="1"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/RevenuefromContractswithCustomersNarrativeDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#RevenuefromContractswithCustomersNarrativeDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/RevenuefromContractswithCustomersNarrativeDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_5ef1fed3-4d63-4952-af61-a98f53c0f37e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_237fa4c5-a766-4a30-a730-cb4953231fbf" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_5ef1fed3-4d63-4952-af61-a98f53c0f37e" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_237fa4c5-a766-4a30-a730-cb4953231fbf" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_1a263610-3a42-490c-8fec-b5d26a33748b" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_237fa4c5-a766-4a30-a730-cb4953231fbf" xlink:to="loc_srt_RangeAxis_1a263610-3a42-490c-8fec-b5d26a33748b" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_1a263610-3a42-490c-8fec-b5d26a33748b_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_1a263610-3a42-490c-8fec-b5d26a33748b" xlink:to="loc_srt_RangeMember_1a263610-3a42-490c-8fec-b5d26a33748b_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_688b3da8-3419-4186-a88a-c4024658765c" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_1a263610-3a42-490c-8fec-b5d26a33748b" xlink:to="loc_srt_RangeMember_688b3da8-3419-4186-a88a-c4024658765c" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_ec386e5f-8cde-4fad-a088-9655f2ee4e47" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MinimumMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_688b3da8-3419-4186-a88a-c4024658765c" xlink:to="loc_srt_MinimumMember_ec386e5f-8cde-4fad-a088-9655f2ee4e47" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_00d05477-9fcb-44b1-8d56-4662b3374afc" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MaximumMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_688b3da8-3419-4186-a88a-c4024658765c" xlink:to="loc_srt_MaximumMember_00d05477-9fcb-44b1-8d56-4662b3374afc" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_SupplierPaymentTimingPeriod_ad52409d-20c1-4efb-b60c-bc07b890d9a4" xlink:href="ecvt-20231231.xsd#ecvt_SupplierPaymentTimingPeriod"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_5ef1fed3-4d63-4952-af61-a98f53c0f37e" xlink:to="loc_ecvt_SupplierPaymentTimingPeriod_ad52409d-20c1-4efb-b60c-bc07b890d9a4" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerAssetNet_d3a8aa97-c81b-4377-8f52-60bcce2a7590" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ContractWithCustomerAssetNet"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_5ef1fed3-4d63-4952-af61-a98f53c0f37e" xlink:to="loc_us-gaap_ContractWithCustomerAssetNet_d3a8aa97-c81b-4377-8f52-60bcce2a7590" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiability_c2933e28-0f31-48cc-8bf3-063281ba41a3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ContractWithCustomerLiability"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_5ef1fed3-4d63-4952-af61-a98f53c0f37e" xlink:to="loc_us-gaap_ContractWithCustomerLiability_c2933e28-0f31-48cc-8bf3-063281ba41a3" xlink:type="arc" order="2"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/RevenuefromContractswithCustomersDisaggregatedRevenueDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#RevenuefromContractswithCustomersDisaggregatedRevenueDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/RevenuefromContractswithCustomersDisaggregatedRevenueDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_6c319979-8863-4048-9049-254d02dcc675" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_7d47391f-78e7-4b8a-a2ba-0b249da907c0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisaggregationOfRevenueTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_6c319979-8863-4048-9049-254d02dcc675" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_7d47391f-78e7-4b8a-a2ba-0b249da907c0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_831d1e18-91a3-4905-ae2f-057c0042b9b9" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_7d47391f-78e7-4b8a-a2ba-0b249da907c0" xlink:to="loc_srt_ConsolidationItemsAxis_831d1e18-91a3-4905-ae2f-057c0042b9b9" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_831d1e18-91a3-4905-ae2f-057c0042b9b9_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_831d1e18-91a3-4905-ae2f-057c0042b9b9" xlink:to="loc_srt_ConsolidationItemsDomain_831d1e18-91a3-4905-ae2f-057c0042b9b9_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_4a691efc-a4a9-4161-bab9-da62204c3a44" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_831d1e18-91a3-4905-ae2f-057c0042b9b9" xlink:to="loc_srt_ConsolidationItemsDomain_4a691efc-a4a9-4161-bab9-da62204c3a44" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_3f4ffd08-3d2b-4549-9af8-f5651e002ab4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OperatingSegmentsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_4a691efc-a4a9-4161-bab9-da62204c3a44" xlink:to="loc_us-gaap_OperatingSegmentsMember_3f4ffd08-3d2b-4549-9af8-f5651e002ab4" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_a284045b-3145-412b-9da3-1be5dcdbfaef" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementBusinessSegmentsAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_7d47391f-78e7-4b8a-a2ba-0b249da907c0" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_a284045b-3145-412b-9da3-1be5dcdbfaef" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_a284045b-3145-412b-9da3-1be5dcdbfaef_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_a284045b-3145-412b-9da3-1be5dcdbfaef" xlink:to="loc_us-gaap_SegmentDomain_a284045b-3145-412b-9da3-1be5dcdbfaef_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_9a74e96f-bdf3-4e77-9858-07e5fa3e8789" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_a284045b-3145-412b-9da3-1be5dcdbfaef" xlink:to="loc_us-gaap_SegmentDomain_9a74e96f-bdf3-4e77-9858-07e5fa3e8789" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_EcoservicesMember_2ad49d38-8fb5-4f70-ad7c-8cbc83731b0a" xlink:href="ecvt-20231231.xsd#ecvt_EcoservicesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_9a74e96f-bdf3-4e77-9858-07e5fa3e8789" xlink:to="loc_ecvt_EcoservicesMember_2ad49d38-8fb5-4f70-ad7c-8cbc83731b0a" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_AdvancedMaterialsAndCatalystsMember_22d56a18-720e-4857-af77-e8c427f28ec3" xlink:href="ecvt-20231231.xsd#ecvt_AdvancedMaterialsAndCatalystsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_9a74e96f-bdf3-4e77-9858-07e5fa3e8789" xlink:to="loc_ecvt_AdvancedMaterialsAndCatalystsMember_22d56a18-720e-4857-af77-e8c427f28ec3" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_365d9b00-9a42-4101-a186-0edf4222fd95" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ProductOrServiceAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_7d47391f-78e7-4b8a-a2ba-0b249da907c0" xlink:to="loc_srt_ProductOrServiceAxis_365d9b00-9a42-4101-a186-0edf4222fd95" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_365d9b00-9a42-4101-a186-0edf4222fd95_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ProductsAndServicesDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_365d9b00-9a42-4101-a186-0edf4222fd95" xlink:to="loc_srt_ProductsAndServicesDomain_365d9b00-9a42-4101-a186-0edf4222fd95_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_8c3972a7-205c-4442-9d33-562a563f1fc6" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ProductsAndServicesDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_365d9b00-9a42-4101-a186-0edf4222fd95" xlink:to="loc_srt_ProductsAndServicesDomain_8c3972a7-205c-4442-9d33-562a563f1fc6" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_CleanFuelsEmissionControlsOtherMember_f61f93c5-7935-4d72-b2e2-c2126b9fc00d" xlink:href="ecvt-20231231.xsd#ecvt_CleanFuelsEmissionControlsOtherMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_8c3972a7-205c-4442-9d33-562a563f1fc6" xlink:to="loc_ecvt_CleanFuelsEmissionControlsOtherMember_f61f93c5-7935-4d72-b2e2-c2126b9fc00d" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PackagingAndEngineeringPlasticsMember_086e9d6b-e618-44ff-a65a-927471c81d52" xlink:href="ecvt-20231231.xsd#ecvt_PackagingAndEngineeringPlasticsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_8c3972a7-205c-4442-9d33-562a563f1fc6" xlink:to="loc_ecvt_PackagingAndEngineeringPlasticsMember_086e9d6b-e618-44ff-a65a-927471c81d52" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_RegenerationAndTreatmentServicesMember_6abc6fb7-214b-4e45-bc23-e2caf8f37960" xlink:href="ecvt-20231231.xsd#ecvt_RegenerationAndTreatmentServicesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_8c3972a7-205c-4442-9d33-562a563f1fc6" xlink:to="loc_ecvt_RegenerationAndTreatmentServicesMember_6abc6fb7-214b-4e45-bc23-e2caf8f37960" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_IndustrialMiningAutomotiveMember_3307fe91-da43-49b9-ad66-2f88294e059f" xlink:href="ecvt-20231231.xsd#ecvt_IndustrialMiningAutomotiveMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_8c3972a7-205c-4442-9d33-562a563f1fc6" xlink:to="loc_ecvt_IndustrialMiningAutomotiveMember_3307fe91-da43-49b9-ad66-2f88294e059f" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_02c53ed7-4741-428e-8c01-ef18ff8c0b8e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_6c319979-8863-4048-9049-254d02dcc675" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_02c53ed7-4741-428e-8c01-ef18ff8c0b8e" xlink:type="arc" order="0"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/FairValueMeasurementsAssetsandLiabilitiesMeasuredatFairValueonaRecurringBasisDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#FairValueMeasurementsAssetsandLiabilitiesMeasuredatFairValueonaRecurringBasisDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/FairValueMeasurementsAssetsandLiabilitiesMeasuredatFairValueonaRecurringBasisDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_21fddc1b-1c16-4631-a5f7-6b887691a1b9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_0b6afea5-5794-439a-8163-ec457feabafe" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_21fddc1b-1c16-4631-a5f7-6b887691a1b9" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_0b6afea5-5794-439a-8163-ec457feabafe" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_e5fae815-86d8-4167-9fbb-2653bace8fa8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_0b6afea5-5794-439a-8163-ec457feabafe" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_e5fae815-86d8-4167-9fbb-2653bace8fa8" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_e5fae815-86d8-4167-9fbb-2653bace8fa8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_e5fae815-86d8-4167-9fbb-2653bace8fa8" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_e5fae815-86d8-4167-9fbb-2653bace8fa8_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_ab18e10e-5718-4f49-b59b-8d5459a8a897" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_e5fae815-86d8-4167-9fbb-2653bace8fa8" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_ab18e10e-5718-4f49-b59b-8d5459a8a897" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_412a8d5d-052b-4dd4-b174-14175ff77673" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_ab18e10e-5718-4f49-b59b-8d5459a8a897" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_412a8d5d-052b-4dd4-b174-14175ff77673" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_c0f0e974-b260-4263-b1bc-1f597c3a8a2d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_0b6afea5-5794-439a-8163-ec457feabafe" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_c0f0e974-b260-4263-b1bc-1f597c3a8a2d" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_c0f0e974-b260-4263-b1bc-1f597c3a8a2d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_c0f0e974-b260-4263-b1bc-1f597c3a8a2d" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_c0f0e974-b260-4263-b1bc-1f597c3a8a2d_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_04d262a2-19c6-45e3-b771-e8c365c5e774" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_c0f0e974-b260-4263-b1bc-1f597c3a8a2d" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_04d262a2-19c6-45e3-b771-e8c365c5e774" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_4771d1b8-f14b-457e-8f6c-a876010863f7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueInputsLevel1Member"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_04d262a2-19c6-45e3-b771-e8c365c5e774" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_4771d1b8-f14b-457e-8f6c-a876010863f7" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_e594070d-f165-4167-9ef3-122c963256da" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueInputsLevel2Member"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_04d262a2-19c6-45e3-b771-e8c365c5e774" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_e594070d-f165-4167-9ef3-122c963256da" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_0b2f89d6-2d6b-4c05-8405-4dc9a3c397a6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueInputsLevel3Member"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_04d262a2-19c6-45e3-b771-e8c365c5e774" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_0b2f89d6-2d6b-4c05-8405-4dc9a3c397a6" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_c31da4fe-fc40-460d-9ec1-d401acde03e5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AssetsAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_21fddc1b-1c16-4631-a5f7-6b887691a1b9" xlink:to="loc_us-gaap_AssetsAbstract_c31da4fe-fc40-460d-9ec1-d401acde03e5" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssets_b95042d4-d6ef-44c8-8587-517e6913f467" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeAssets"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_c31da4fe-fc40-460d-9ec1-d401acde03e5" xlink:to="loc_us-gaap_DerivativeAssets_b95042d4-d6ef-44c8-8587-517e6913f467" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialLiabilitiesFairValueDisclosureAbstract_1ee58b81-5cdc-45ce-9dcb-6dca0b3e7e53" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FinancialLiabilitiesFairValueDisclosureAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_21fddc1b-1c16-4631-a5f7-6b887691a1b9" xlink:to="loc_us-gaap_FinancialLiabilitiesFairValueDisclosureAbstract_1ee58b81-5cdc-45ce-9dcb-6dca0b3e7e53" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilities_ef53bc48-e84b-4c35-a09d-0ebf525d7277" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeLiabilities"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialLiabilitiesFairValueDisclosureAbstract_1ee58b81-5cdc-45ce-9dcb-6dca0b3e7e53" xlink:to="loc_us-gaap_DerivativeLiabilities_ef53bc48-e84b-4c35-a09d-0ebf525d7277" xlink:type="arc" order="1"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/StockholdersEquityComponentsofAOCIDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#StockholdersEquityComponentsofAOCIDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/StockholdersEquityComponentsofAOCIDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_dc5cc0cf-b662-43ec-bcd8-e1fed6bc755f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_327284f6-8901-437e-b923-54a35a74249d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_dc5cc0cf-b662-43ec-bcd8-e1fed6bc755f" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_327284f6-8901-437e-b923-54a35a74249d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_0f7e855d-61c8-4d75-9fe3-e70440bbaf07" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementEquityComponentsAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_327284f6-8901-437e-b923-54a35a74249d" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_0f7e855d-61c8-4d75-9fe3-e70440bbaf07" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_0f7e855d-61c8-4d75-9fe3-e70440bbaf07_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_0f7e855d-61c8-4d75-9fe3-e70440bbaf07" xlink:to="loc_us-gaap_EquityComponentDomain_0f7e855d-61c8-4d75-9fe3-e70440bbaf07_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_06ca5fa2-4b29-4c64-9f5f-94563ab67cad" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_0f7e855d-61c8-4d75-9fe3-e70440bbaf07" xlink:to="loc_us-gaap_EquityComponentDomain_06ca5fa2-4b29-4c64-9f5f-94563ab67cad" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_cdd03d95-de52-4aeb-a03f-b53b38c4cc23" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_06ca5fa2-4b29-4c64-9f5f-94563ab67cad" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_cdd03d95-de52-4aeb-a03f-b53b38c4cc23" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_58e29802-46d9-43f6-b444-f3c3460f7d5f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_cdd03d95-de52-4aeb-a03f-b53b38c4cc23" xlink:to="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_58e29802-46d9-43f6-b444-f3c3460f7d5f" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_bf9057f5-6a20-48eb-a9ec-c4ea52e1332e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_cdd03d95-de52-4aeb-a03f-b53b38c4cc23" xlink:to="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_bf9057f5-6a20-48eb-a9ec-c4ea52e1332e" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedTranslationAdjustmentMember_7fcc05cc-0638-47bf-b015-a69dbc0c1acf" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedTranslationAdjustmentMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_cdd03d95-de52-4aeb-a03f-b53b38c4cc23" xlink:to="loc_us-gaap_AccumulatedTranslationAdjustmentMember_7fcc05cc-0638-47bf-b015-a69dbc0c1acf" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OciNetOfTaxAbstract_3cfea165-48a6-45c7-913c-c71743d4041e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OciNetOfTaxAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_dc5cc0cf-b662-43ec-bcd8-e1fed6bc755f" xlink:to="loc_us-gaap_OciNetOfTaxAbstract_3cfea165-48a6-45c7-913c-c71743d4041e" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AociTaxAttributableToParent_88d8b5cc-2e7c-4ad4-8ff4-548c92b3177c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AociTaxAttributableToParent"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OciNetOfTaxAbstract_3cfea165-48a6-45c7-913c-c71743d4041e" xlink:to="loc_us-gaap_AociTaxAttributableToParent_88d8b5cc-2e7c-4ad4-8ff4-548c92b3177c" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTaxAbstract_9cf405ef-d6b5-4e6f-9482-5a231896964d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTaxAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_dc5cc0cf-b662-43ec-bcd8-e1fed6bc755f" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTaxAbstract_9cf405ef-d6b5-4e6f-9482-5a231896964d" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_949ff2d5-77b5-4e14-ad4f-a0df6984c72d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTaxAbstract_9cf405ef-d6b5-4e6f-9482-5a231896964d" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_949ff2d5-77b5-4e14-ad4f-a0df6984c72d" xlink:type="arc" order="1"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/StockholdersEquityPretaxandAftertaxComponentsofOtherComprehensiveIncomeLossDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#StockholdersEquityPretaxandAftertaxComponentsofOtherComprehensiveIncomeLossDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/StockholdersEquityPretaxandAftertaxComponentsofOtherComprehensiveIncomeLossDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_b29c2f2e-74d9-4ada-b626-f4f97c0e6afc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_0b0f8dcc-c413-4afe-9c80-b790134494b2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_b29c2f2e-74d9-4ada-b626-f4f97c0e6afc" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_0b0f8dcc-c413-4afe-9c80-b790134494b2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_2983b096-ad38-4f18-9d45-114b79175560" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementEquityComponentsAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_0b0f8dcc-c413-4afe-9c80-b790134494b2" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_2983b096-ad38-4f18-9d45-114b79175560" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_2983b096-ad38-4f18-9d45-114b79175560_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_2983b096-ad38-4f18-9d45-114b79175560" xlink:to="loc_us-gaap_EquityComponentDomain_2983b096-ad38-4f18-9d45-114b79175560_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_5719fee6-946f-4ebf-972d-fe300a933cf9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_2983b096-ad38-4f18-9d45-114b79175560" xlink:to="loc_us-gaap_EquityComponentDomain_5719fee6-946f-4ebf-972d-fe300a933cf9" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetGainLossIncludingPortionAttributableToNoncontrollingInterestMember_6035a8c8-39b9-44c1-94ed-293e813e28e4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetGainLossIncludingPortionAttributableToNoncontrollingInterestMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_5719fee6-946f-4ebf-972d-fe300a933cf9" xlink:to="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetGainLossIncludingPortionAttributableToNoncontrollingInterestMember_6035a8c8-39b9-44c1-94ed-293e813e28e4" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceIncludingPortionAttributableToNoncontrollingInterestMember_0cc883c8-9a8d-4910-b1f7-3fc185745457" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceIncludingPortionAttributableToNoncontrollingInterestMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_5719fee6-946f-4ebf-972d-fe300a933cf9" xlink:to="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceIncludingPortionAttributableToNoncontrollingInterestMember_0cc883c8-9a8d-4910-b1f7-3fc185745457" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentIncludingPortionAttributableToNoncontrollingInterestMember_75fff612-80a3-4ee4-8d91-e7952e9718d8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedDefinedBenefitPlansAdjustmentIncludingPortionAttributableToNoncontrollingInterestMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_5719fee6-946f-4ebf-972d-fe300a933cf9" xlink:to="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentIncludingPortionAttributableToNoncontrollingInterestMember_75fff612-80a3-4ee4-8d91-e7952e9718d8" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedNetGainLossFromCashFlowHedgesIncludingPortionAttributableToNoncontrollingInterestMember_871daed2-6428-40bf-be05-4090b32b612a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedNetGainLossFromCashFlowHedgesIncludingPortionAttributableToNoncontrollingInterestMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_5719fee6-946f-4ebf-972d-fe300a933cf9" xlink:to="loc_us-gaap_AccumulatedNetGainLossFromCashFlowHedgesIncludingPortionAttributableToNoncontrollingInterestMember_871daed2-6428-40bf-be05-4090b32b612a" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedForeignCurrencyAdjustmentIncludingPortionAttributableToNoncontrollingInterestMember_aa635e32-ede3-4f15-80d6-db227ab54ca1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedForeignCurrencyAdjustmentIncludingPortionAttributableToNoncontrollingInterestMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_5719fee6-946f-4ebf-972d-fe300a933cf9" xlink:to="loc_us-gaap_AccumulatedForeignCurrencyAdjustmentIncludingPortionAttributableToNoncontrollingInterestMember_aa635e32-ede3-4f15-80d6-db227ab54ca1" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPortionAttributableToParentAbstract_f579644c-7913-485b-993e-5d826b513348" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPortionAttributableToParentAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_b29c2f2e-74d9-4ada-b626-f4f97c0e6afc" xlink:to="loc_us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPortionAttributableToParentAbstract_f579644c-7913-485b-993e-5d826b513348" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeTax_645aa46e-6c2b-485c-a790-80a8bd3582d2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPortionAttributableToParentAbstract_f579644c-7913-485b-993e-5d826b513348" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeTax_645aa46e-6c2b-485c-a790-80a8bd3582d2" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossTax_e91da947-aab0-4316-bd6d-d797496e934a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPortionAttributableToParentAbstract_f579644c-7913-485b-993e-5d826b513348" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossTax_e91da947-aab0-4316-bd6d-d797496e934a" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_344a47f5-47ae-40ea-9e43-ab6ff27fb467" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPortionAttributableToParentAbstract_f579644c-7913-485b-993e-5d826b513348" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_344a47f5-47ae-40ea-9e43-ab6ff27fb467" xlink:type="arc" order="3"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/StockholdersEquityChangebyComponentDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#StockholdersEquityChangebyComponentDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/StockholdersEquityChangebyComponentDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_62d7fa82-4549-4367-81c3-57686d6412d9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_389e1642-424e-4343-acc3-d72b3952ee86" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_62d7fa82-4549-4367-81c3-57686d6412d9" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_389e1642-424e-4343-acc3-d72b3952ee86" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_e490db38-373f-43fe-94ef-d270c630dcb1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementEquityComponentsAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_389e1642-424e-4343-acc3-d72b3952ee86" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_e490db38-373f-43fe-94ef-d270c630dcb1" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_e490db38-373f-43fe-94ef-d270c630dcb1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_e490db38-373f-43fe-94ef-d270c630dcb1" xlink:to="loc_us-gaap_EquityComponentDomain_e490db38-373f-43fe-94ef-d270c630dcb1_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_fbc29686-2e8b-4a60-baa8-4a4ed34a5c9b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_e490db38-373f-43fe-94ef-d270c630dcb1" xlink:to="loc_us-gaap_EquityComponentDomain_fbc29686-2e8b-4a60-baa8-4a4ed34a5c9b" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AociIncludingPortionAttributableToNoncontrollingInterestMember_df5182de-aa09-4fdd-99da-7d79f45839d1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AociIncludingPortionAttributableToNoncontrollingInterestMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_fbc29686-2e8b-4a60-baa8-4a4ed34a5c9b" xlink:to="loc_us-gaap_AociIncludingPortionAttributableToNoncontrollingInterestMember_df5182de-aa09-4fdd-99da-7d79f45839d1" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentIncludingPortionAttributableToNoncontrollingInterestMember_6530a674-56b9-4dc8-924f-164e7c728f21" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedDefinedBenefitPlansAdjustmentIncludingPortionAttributableToNoncontrollingInterestMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AociIncludingPortionAttributableToNoncontrollingInterestMember_df5182de-aa09-4fdd-99da-7d79f45839d1" xlink:to="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentIncludingPortionAttributableToNoncontrollingInterestMember_6530a674-56b9-4dc8-924f-164e7c728f21" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedNetGainLossFromCashFlowHedgesIncludingPortionAttributableToNoncontrollingInterestMember_0bf9f26d-ebcf-40f5-8f61-460b8b8dcb08" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedNetGainLossFromCashFlowHedgesIncludingPortionAttributableToNoncontrollingInterestMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AociIncludingPortionAttributableToNoncontrollingInterestMember_df5182de-aa09-4fdd-99da-7d79f45839d1" xlink:to="loc_us-gaap_AccumulatedNetGainLossFromCashFlowHedgesIncludingPortionAttributableToNoncontrollingInterestMember_0bf9f26d-ebcf-40f5-8f61-460b8b8dcb08" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedForeignCurrencyAdjustmentIncludingPortionAttributableToNoncontrollingInterestMember_f73279e3-2b0f-47a7-a917-a544178c40b4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedForeignCurrencyAdjustmentIncludingPortionAttributableToNoncontrollingInterestMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AociIncludingPortionAttributableToNoncontrollingInterestMember_df5182de-aa09-4fdd-99da-7d79f45839d1" xlink:to="loc_us-gaap_AccumulatedForeignCurrencyAdjustmentIncludingPortionAttributableToNoncontrollingInterestMember_f73279e3-2b0f-47a7-a917-a544178c40b4" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_fb6da372-d811-4982-ab01-5234a8a4db71" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_62d7fa82-4549-4367-81c3-57686d6412d9" xlink:to="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_fb6da372-d811-4982-ab01-5234a8a4db71" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_6b7244fe-a527-4477-976f-d5450d5c3f9e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_fb6da372-d811-4982-ab01-5234a8a4db71" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_6b7244fe-a527-4477-976f-d5450d5c3f9e" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax_3e7195f0-c920-421b-a39b-f13c6f36d259" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_fb6da372-d811-4982-ab01-5234a8a4db71" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax_3e7195f0-c920-421b-a39b-f13c6f36d259" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax_9ebc9923-e362-4708-ad66-2a5ee14ca87b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_fb6da372-d811-4982-ab01-5234a8a4db71" xlink:to="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax_9ebc9923-e362-4708-ad66-2a5ee14ca87b" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_0401cb20-bbd7-44c1-bae5-332e13b30689" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_fb6da372-d811-4982-ab01-5234a8a4db71" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_0401cb20-bbd7-44c1-bae5-332e13b30689" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_bc67cbb4-971d-444d-bb91-e8816a0eb1ec" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/StockholdersEquityReclassificationsoutofAOCIDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#StockholdersEquityReclassificationsoutofAOCIDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/StockholdersEquityReclassificationsoutofAOCIDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_72e8d2f5-a275-4665-9eec-a88193ce1cdb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_0c63a44a-c15f-4919-813a-7ae517f2bf61" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_72e8d2f5-a275-4665-9eec-a88193ce1cdb" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_0c63a44a-c15f-4919-813a-7ae517f2bf61" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_2df03914-f4a6-4b9b-81f6-8b1e835dd6a4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementEquityComponentsAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_0c63a44a-c15f-4919-813a-7ae517f2bf61" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_2df03914-f4a6-4b9b-81f6-8b1e835dd6a4" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_2df03914-f4a6-4b9b-81f6-8b1e835dd6a4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_2df03914-f4a6-4b9b-81f6-8b1e835dd6a4" xlink:to="loc_us-gaap_EquityComponentDomain_2df03914-f4a6-4b9b-81f6-8b1e835dd6a4_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_caca6eb3-fa1b-48a9-8ca4-7d2009372753" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_2df03914-f4a6-4b9b-81f6-8b1e835dd6a4" xlink:to="loc_us-gaap_EquityComponentDomain_caca6eb3-fa1b-48a9-8ca4-7d2009372753" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_86b1c0eb-c7cf-43f2-b6df-c2994489f168" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_caca6eb3-fa1b-48a9-8ca4-7d2009372753" xlink:to="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_86b1c0eb-c7cf-43f2-b6df-c2994489f168" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember_6bfa5707-89af-42eb-a4c4-21ff377ba471" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_86b1c0eb-c7cf-43f2-b6df-c2994489f168" xlink:to="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember_6bfa5707-89af-42eb-a4c4-21ff377ba471" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember_896bc985-a79d-4c55-a5f1-b96898636f41" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_86b1c0eb-c7cf-43f2-b6df-c2994489f168" xlink:to="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember_896bc985-a79d-4c55-a5f1-b96898636f41" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_224f3f8d-c366-475d-9abb-4c2cba01904f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_caca6eb3-fa1b-48a9-8ca4-7d2009372753" xlink:to="loc_us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember_224f3f8d-c366-475d-9abb-4c2cba01904f" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_4604d3d9-0fea-4ef3-8a82-14c28e30bcdd" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_0c63a44a-c15f-4919-813a-7ae517f2bf61" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_4604d3d9-0fea-4ef3-8a82-14c28e30bcdd" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_4604d3d9-0fea-4ef3-8a82-14c28e30bcdd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_4604d3d9-0fea-4ef3-8a82-14c28e30bcdd" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_4604d3d9-0fea-4ef3-8a82-14c28e30bcdd_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_45ba9446-b978-48d4-bbbc-71e6a1d8b393" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_4604d3d9-0fea-4ef3-8a82-14c28e30bcdd" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_45ba9446-b978-48d4-bbbc-71e6a1d8b393" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_8d5c0f56-44a3-4558-85b8-2ebb4c68ecea" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_45ba9446-b978-48d4-bbbc-71e6a1d8b393" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_8d5c0f56-44a3-4558-85b8-2ebb4c68ecea" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_1fd6f62b-d933-4450-b946-9a200ca9794e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_0c63a44a-c15f-4919-813a-7ae517f2bf61" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_1fd6f62b-d933-4450-b946-9a200ca9794e" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_1fd6f62b-d933-4450-b946-9a200ca9794e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeContractTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_1fd6f62b-d933-4450-b946-9a200ca9794e" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_1fd6f62b-d933-4450-b946-9a200ca9794e_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_809bfb28-b5b4-4ae3-8813-774123d2cd75" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeContractTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_1fd6f62b-d933-4450-b946-9a200ca9794e" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_809bfb28-b5b4-4ae3-8813-774123d2cd75" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateCapMember_0ced135e-3937-4934-8350-0aa766fe2d43" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_InterestRateCapMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_809bfb28-b5b4-4ae3-8813-774123d2cd75" xlink:to="loc_us-gaap_InterestRateCapMember_0ced135e-3937-4934-8350-0aa766fe2d43" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonoperatingIncomeExpense_b7f36bc2-bbe0-4066-a19b-7999472702f8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NonoperatingIncomeExpense"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_72e8d2f5-a275-4665-9eec-a88193ce1cdb" xlink:to="loc_us-gaap_NonoperatingIncomeExpense_b7f36bc2-bbe0-4066-a19b-7999472702f8" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_5a7f6b2c-7d0c-4481-a993-8de79bedc596" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_InterestExpense"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_72e8d2f5-a275-4665-9eec-a88193ce1cdb" xlink:to="loc_us-gaap_InterestExpense_5a7f6b2c-7d0c-4481-a993-8de79bedc596" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_790f4131-68aa-4ac2-bede-0c1557636662" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_72e8d2f5-a275-4665-9eec-a88193ce1cdb" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_790f4131-68aa-4ac2-bede-0c1557636662" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_61d0b611-c223-4c4b-8cea-077ab23517c2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeTaxExpenseBenefit"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_72e8d2f5-a275-4665-9eec-a88193ce1cdb" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_61d0b611-c223-4c4b-8cea-077ab23517c2" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_77f8e3d2-b6af-49ed-b25d-2633965f8a00" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ProfitLoss"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_72e8d2f5-a275-4665-9eec-a88193ce1cdb" xlink:to="loc_us-gaap_ProfitLoss_77f8e3d2-b6af-49ed-b25d-2633965f8a00" xlink:type="arc" order="4"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/StockholdersEquityTreasuryStockRepurchasesDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#StockholdersEquityTreasuryStockRepurchasesDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/StockholdersEquityTreasuryStockRepurchasesDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityClassOfTreasuryStockLineItems_31eefc96-3bcf-4076-bd7b-47b2bd52a1a4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityClassOfTreasuryStockLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfTreasuryStockTable_2f66c0a9-c458-4410-a357-a47da6f9086a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfTreasuryStockTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_31eefc96-3bcf-4076-bd7b-47b2bd52a1a4" xlink:to="loc_us-gaap_ClassOfTreasuryStockTable_2f66c0a9-c458-4410-a357-a47da6f9086a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareRepurchaseProgramAxis_8f3877c0-b89e-4d8d-a8b7-a67447a50208" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareRepurchaseProgramAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ClassOfTreasuryStockTable_2f66c0a9-c458-4410-a357-a47da6f9086a" xlink:to="loc_us-gaap_ShareRepurchaseProgramAxis_8f3877c0-b89e-4d8d-a8b7-a67447a50208" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_8f3877c0-b89e-4d8d-a8b7-a67447a50208_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareRepurchaseProgramDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_8f3877c0-b89e-4d8d-a8b7-a67447a50208" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_8f3877c0-b89e-4d8d-a8b7-a67447a50208_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_20f8789a-2304-4e83-917f-6175a4197f2c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareRepurchaseProgramDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_8f3877c0-b89e-4d8d-a8b7-a67447a50208" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_20f8789a-2304-4e83-917f-6175a4197f2c" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_March2020StockRepurchaseProgramMember_4ff6aab6-f87f-40fa-9960-fdc48fa8e5d4" xlink:href="ecvt-20231231.xsd#ecvt_March2020StockRepurchaseProgramMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareRepurchaseProgramDomain_20f8789a-2304-4e83-917f-6175a4197f2c" xlink:to="loc_ecvt_March2020StockRepurchaseProgramMember_4ff6aab6-f87f-40fa-9960-fdc48fa8e5d4" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_April2022StockRepurchaseProgramMember_7d945c94-fc50-455c-817c-c5b8bd27e6ba" xlink:href="ecvt-20231231.xsd#ecvt_April2022StockRepurchaseProgramMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareRepurchaseProgramDomain_20f8789a-2304-4e83-917f-6175a4197f2c" xlink:to="loc_ecvt_April2022StockRepurchaseProgramMember_7d945c94-fc50-455c-817c-c5b8bd27e6ba" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_ShareRepurchaseMethodAxis_0336f2fc-b596-4550-ac69-77314776effc" xlink:href="ecvt-20231231.xsd#ecvt_ShareRepurchaseMethodAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ClassOfTreasuryStockTable_2f66c0a9-c458-4410-a357-a47da6f9086a" xlink:to="loc_ecvt_ShareRepurchaseMethodAxis_0336f2fc-b596-4550-ac69-77314776effc" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_ShareRepurchaseMethodDomain_0336f2fc-b596-4550-ac69-77314776effc_default" xlink:href="ecvt-20231231.xsd#ecvt_ShareRepurchaseMethodDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_ecvt_ShareRepurchaseMethodAxis_0336f2fc-b596-4550-ac69-77314776effc" xlink:to="loc_ecvt_ShareRepurchaseMethodDomain_0336f2fc-b596-4550-ac69-77314776effc_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_ShareRepurchaseMethodDomain_ea1352e0-24de-424e-a8f8-1efe1947c906" xlink:href="ecvt-20231231.xsd#ecvt_ShareRepurchaseMethodDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ecvt_ShareRepurchaseMethodAxis_0336f2fc-b596-4550-ac69-77314776effc" xlink:to="loc_ecvt_ShareRepurchaseMethodDomain_ea1352e0-24de-424e-a8f8-1efe1947c906" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_OpenMarketMember_5f8cee4e-b5af-4504-b2b2-46ea5526dd96" xlink:href="ecvt-20231231.xsd#ecvt_OpenMarketMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecvt_ShareRepurchaseMethodDomain_ea1352e0-24de-424e-a8f8-1efe1947c906" xlink:to="loc_ecvt_OpenMarketMember_5f8cee4e-b5af-4504-b2b2-46ea5526dd96" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_SecondaryOfferingMember_4c01aa09-da75-416c-9809-9ab7e42432aa" xlink:href="ecvt-20231231.xsd#ecvt_SecondaryOfferingMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecvt_ShareRepurchaseMethodDomain_ea1352e0-24de-424e-a8f8-1efe1947c906" xlink:to="loc_ecvt_SecondaryOfferingMember_4c01aa09-da75-416c-9809-9ab7e42432aa" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchaseProgramAuthorizedAmount1_6d6ad130-ab00-4583-8626-584fcc67300b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockRepurchaseProgramAuthorizedAmount1"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_31eefc96-3bcf-4076-bd7b-47b2bd52a1a4" xlink:to="loc_us-gaap_StockRepurchaseProgramAuthorizedAmount1_6d6ad130-ab00-4583-8626-584fcc67300b" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchaseProgramPeriodInForce1_e8395f98-f022-404d-9966-349f72ce79d3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockRepurchaseProgramPeriodInForce1"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_31eefc96-3bcf-4076-bd7b-47b2bd52a1a4" xlink:to="loc_us-gaap_StockRepurchaseProgramPeriodInForce1_e8395f98-f022-404d-9966-349f72ce79d3" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockSharesAcquired_eea16f67-538a-460e-aa73-9dce1f355b5b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TreasuryStockSharesAcquired"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_31eefc96-3bcf-4076-bd7b-47b2bd52a1a4" xlink:to="loc_us-gaap_TreasuryStockSharesAcquired_eea16f67-538a-460e-aa73-9dce1f355b5b" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_48ac5bfa-df19-4ebc-8a37-b9ed3be067b5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TreasuryStockAcquiredAverageCostPerShare"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_31eefc96-3bcf-4076-bd7b-47b2bd52a1a4" xlink:to="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_48ac5bfa-df19-4ebc-8a37-b9ed3be067b5" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonValue_6f16809b-459b-455e-9b9a-398876290ecb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TreasuryStockCommonValue"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_31eefc96-3bcf-4076-bd7b-47b2bd52a1a4" xlink:to="loc_us-gaap_TreasuryStockCommonValue_6f16809b-459b-455e-9b9a-398876290ecb" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_34374b4a-3c49-4a1d-8370-75e557fe76cf" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_31eefc96-3bcf-4076-bd7b-47b2bd52a1a4" xlink:to="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_34374b4a-3c49-4a1d-8370-75e557fe76cf" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_ExciseTaxesPayable_6701250e-7e08-4ed0-9f47-c72a7b6284ea" xlink:href="ecvt-20231231.xsd#ecvt_ExciseTaxesPayable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_31eefc96-3bcf-4076-bd7b-47b2bd52a1a4" xlink:to="loc_ecvt_ExciseTaxesPayable_6701250e-7e08-4ed0-9f47-c72a7b6284ea" xlink:type="arc" order="6"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_8f5f67b1-4493-4a4c-b1ce-a947285e5f3a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_31eefc96-3bcf-4076-bd7b-47b2bd52a1a4" xlink:to="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_8f5f67b1-4493-4a4c-b1ce-a947285e5f3a" xlink:type="arc" order="7"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_ee6ece26-ec85-4c92-a8e5-beff452b4a8d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_31eefc96-3bcf-4076-bd7b-47b2bd52a1a4" xlink:to="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_ee6ece26-ec85-4c92-a8e5-beff452b4a8d" xlink:type="arc" order="8"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/InventoriesNetNarrativesDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#InventoriesNetNarrativesDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/InventoriesNetNarrativesDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryLineItems_3afc3543-c0d2-459e-aeb9-5b933a246798" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_InventoryLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryCurrentTable_0d67a6ad-00b1-48e6-ba03-4be93dc7f329" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_InventoryCurrentTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_InventoryLineItems_3afc3543-c0d2-459e-aeb9-5b933a246798" xlink:to="loc_us-gaap_InventoryCurrentTable_0d67a6ad-00b1-48e6-ba03-4be93dc7f329" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_495f6e4c-fd89-44b0-bcc0-722d2e9b0100" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessAcquisitionAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_InventoryCurrentTable_0d67a6ad-00b1-48e6-ba03-4be93dc7f329" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_495f6e4c-fd89-44b0-bcc0-722d2e9b0100" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_495f6e4c-fd89-44b0-bcc0-722d2e9b0100_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_495f6e4c-fd89-44b0-bcc0-722d2e9b0100" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_495f6e4c-fd89-44b0-bcc0-722d2e9b0100_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0e0f6c9a-6ee3-4997-a10c-a47233491c32" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_495f6e4c-fd89-44b0-bcc0-722d2e9b0100" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0e0f6c9a-6ee3-4997-a10c-a47233491c32" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_BusinessCombinationsMember_6c535b5e-c1e7-4cbc-8476-4bf0e1579f4e" xlink:href="ecvt-20231231.xsd#ecvt_BusinessCombinationsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0e0f6c9a-6ee3-4997-a10c-a47233491c32" xlink:to="loc_ecvt_BusinessCombinationsMember_6c535b5e-c1e7-4cbc-8476-4bf0e1579f4e" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_InventoryValuationInFIFOLowerThanLIFO_e618a1bc-5309-43eb-b1d6-0d950a8d8517" xlink:href="ecvt-20231231.xsd#ecvt_InventoryValuationInFIFOLowerThanLIFO"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InventoryLineItems_3afc3543-c0d2-459e-aeb9-5b933a246798" xlink:to="loc_ecvt_InventoryValuationInFIFOLowerThanLIFO_e618a1bc-5309-43eb-b1d6-0d950a8d8517" xlink:type="arc" order="0"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/InvestmentsinAffiliatedCompaniesNarrativesDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#InvestmentsinAffiliatedCompaniesNarrativesDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/InvestmentsinAffiliatedCompaniesNarrativesDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_97758a90-e96a-48de-a153-f549136d79f5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessAcquisitionLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_4286570d-5a25-4911-9a79-83b494b55bae" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_97758a90-e96a-48de-a153-f549136d79f5" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_4286570d-5a25-4911-9a79-83b494b55bae" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_9ea0d51f-4370-47dc-8f3f-eeae48eb7080" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessAcquisitionAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_4286570d-5a25-4911-9a79-83b494b55bae" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_9ea0d51f-4370-47dc-8f3f-eeae48eb7080" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9ea0d51f-4370-47dc-8f3f-eeae48eb7080_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_9ea0d51f-4370-47dc-8f3f-eeae48eb7080" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9ea0d51f-4370-47dc-8f3f-eeae48eb7080_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_f43e6a36-c59b-4c1a-b157-3b9655534da9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_9ea0d51f-4370-47dc-8f3f-eeae48eb7080" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_f43e6a36-c59b-4c1a-b157-3b9655534da9" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PQHoldingsEcoServicesMember_e1a58693-1028-4e9b-9d9c-d0e2982b1772" xlink:href="ecvt-20231231.xsd#ecvt_PQHoldingsEcoServicesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_f43e6a36-c59b-4c1a-b157-3b9655534da9" xlink:to="loc_ecvt_PQHoldingsEcoServicesMember_e1a58693-1028-4e9b-9d9c-d0e2982b1772" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_57013b76-419a-4288-bd67-a06b0551ad82" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_4286570d-5a25-4911-9a79-83b494b55bae" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_57013b76-419a-4288-bd67-a06b0551ad82" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_57013b76-419a-4288-bd67-a06b0551ad82_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RelatedPartyDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_57013b76-419a-4288-bd67-a06b0551ad82" xlink:to="loc_us-gaap_RelatedPartyDomain_57013b76-419a-4288-bd67-a06b0551ad82_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_a0db093e-6b84-4276-8bf9-c8f8190db21b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RelatedPartyDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_57013b76-419a-4288-bd67-a06b0551ad82" xlink:to="loc_us-gaap_RelatedPartyDomain_a0db093e-6b84-4276-8bf9-c8f8190db21b" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_2c5de889-212c-4309-aba0-049786bee728" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RelatedPartyMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_a0db093e-6b84-4276-8bf9-c8f8190db21b" xlink:to="loc_us-gaap_RelatedPartyMember_2c5de889-212c-4309-aba0-049786bee728" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentDifferenceBetweenCarryingAmountAndUnderlyingEquity_722cd957-1289-439d-9d53-61e26a86fa03" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityMethodInvestmentDifferenceBetweenCarryingAmountAndUnderlyingEquity"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_97758a90-e96a-48de-a153-f549136d79f5" xlink:to="loc_us-gaap_EquityMethodInvestmentDifferenceBetweenCarryingAmountAndUnderlyingEquity_722cd957-1289-439d-9d53-61e26a86fa03" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_EquityMethodInvestmentsChargesRelatedtoPurchaseAccounting_f6925ab6-c222-4aae-b58d-832128b13a6d" xlink:href="ecvt-20231231.xsd#ecvt_EquityMethodInvestmentsChargesRelatedtoPurchaseAccounting"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_97758a90-e96a-48de-a153-f549136d79f5" xlink:to="loc_ecvt_EquityMethodInvestmentsChargesRelatedtoPurchaseAccounting_f6925ab6-c222-4aae-b58d-832128b13a6d" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNet_7a24a763-5905-40e0-ab7e-c927370969c1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccountsReceivableNet"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_97758a90-e96a-48de-a153-f549136d79f5" xlink:to="loc_us-gaap_AccountsReceivableNet_7a24a763-5905-40e0-ab7e-c927370969c1" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrent_e31a24a7-60ca-4867-9cc0-2e0b3d65d73c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccountsPayableCurrent"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_97758a90-e96a-48de-a153-f549136d79f5" xlink:to="loc_us-gaap_AccountsPayableCurrent_e31a24a7-60ca-4867-9cc0-2e0b3d65d73c" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_9faa2c1d-05b6-4747-94a9-39a74d2bd3f0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_Revenues"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_97758a90-e96a-48de-a153-f549136d79f5" xlink:to="loc_us-gaap_Revenues_9faa2c1d-05b6-4747-94a9-39a74d2bd3f0" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionPurchasesFromRelatedParty_ec844fbb-b2d4-4cc6-8b32-3c831a2874ec" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RelatedPartyTransactionPurchasesFromRelatedParty"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_97758a90-e96a-48de-a153-f549136d79f5" xlink:to="loc_us-gaap_RelatedPartyTransactionPurchasesFromRelatedParty_ec844fbb-b2d4-4cc6-8b32-3c831a2874ec" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_RealEstateTaxAbatementTerm_58899870-b634-4e7b-aea2-45815ce58e99" xlink:href="ecvt-20231231.xsd#ecvt_RealEstateTaxAbatementTerm"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_97758a90-e96a-48de-a153-f549136d79f5" xlink:to="loc_ecvt_RealEstateTaxAbatementTerm_58899870-b634-4e7b-aea2-45815ce58e99" xlink:type="arc" order="6"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_RealEstateTaxAbatementPercent_dc9458d9-13cb-416c-89fb-d12171a81b66" xlink:href="ecvt-20231231.xsd#ecvt_RealEstateTaxAbatementPercent"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_97758a90-e96a-48de-a153-f549136d79f5" xlink:to="loc_ecvt_RealEstateTaxAbatementPercent_dc9458d9-13cb-416c-89fb-d12171a81b66" xlink:type="arc" order="7"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/InvestmentsinAffiliatedCompaniesOwnershipPercentageDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#InvestmentsinAffiliatedCompaniesOwnershipPercentageDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/InvestmentsinAffiliatedCompaniesOwnershipPercentageDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_2e114c0e-3de3-4a44-8de2-abfc585c27b5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_9781344f-4019-4736-bb73-502174c6e497" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_2e114c0e-3de3-4a44-8de2-abfc585c27b5" xlink:to="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_9781344f-4019-4736-bb73-502174c6e497" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_04d0c883-384b-4423-bdf9-ace0df639625" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_9781344f-4019-4736-bb73-502174c6e497" xlink:to="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_04d0c883-384b-4423-bdf9-ace0df639625" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_04d0c883-384b-4423-bdf9-ace0df639625_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_EquityMethodInvesteeNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_04d0c883-384b-4423-bdf9-ace0df639625" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_04d0c883-384b-4423-bdf9-ace0df639625_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_e384b393-6710-4caa-bb7e-e10990b35cd7" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_EquityMethodInvesteeNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_04d0c883-384b-4423-bdf9-ace0df639625" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_e384b393-6710-4caa-bb7e-e10990b35cd7" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_ZeolystInternationalMember_25ebd517-8875-4bbd-8e46-7090530272a4" xlink:href="ecvt-20231231.xsd#ecvt_ZeolystInternationalMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_e384b393-6710-4caa-bb7e-e10990b35cd7" xlink:to="loc_ecvt_ZeolystInternationalMember_25ebd517-8875-4bbd-8e46-7090530272a4" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_ZeolystC.V.Member_68f7f538-02a9-4796-a100-a25258e24f9f" xlink:href="ecvt-20231231.xsd#ecvt_ZeolystC.V.Member"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_e384b393-6710-4caa-bb7e-e10990b35cd7" xlink:to="loc_ecvt_ZeolystC.V.Member_68f7f538-02a9-4796-a100-a25258e24f9f" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_a691e22e-e03a-4fc7-8f3f-efa3336f0737" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_2e114c0e-3de3-4a44-8de2-abfc585c27b5" xlink:to="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_a691e22e-e03a-4fc7-8f3f-efa3336f0737" xlink:type="arc" order="0"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/InvestmentsinAffiliatedCompaniesSummarizedBalanceSheetDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#InvestmentsinAffiliatedCompaniesSummarizedBalanceSheetDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/InvestmentsinAffiliatedCompaniesSummarizedBalanceSheetDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_604185e0-eeb1-45d3-bc42-38570f198adc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_692e7312-8b8e-43e0-8d68-972cd48c0784" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_604185e0-eeb1-45d3-bc42-38570f198adc" xlink:to="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_692e7312-8b8e-43e0-8d68-972cd48c0784" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeAxis_5e53c45a-199b-424e-86f8-a1afed215bbe" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_692e7312-8b8e-43e0-8d68-972cd48c0784" xlink:to="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeAxis_5e53c45a-199b-424e-86f8-a1afed215bbe" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeDomain_5e53c45a-199b-424e-86f8-a1afed215bbe_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeAxis_5e53c45a-199b-424e-86f8-a1afed215bbe" xlink:to="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeDomain_5e53c45a-199b-424e-86f8-a1afed215bbe_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeDomain_c1438bb2-9458-448e-b28f-4d8e49f86971" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeAxis_5e53c45a-199b-424e-86f8-a1afed215bbe" xlink:to="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeDomain_c1438bb2-9458-448e-b28f-4d8e49f86971" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeOrGroupOfInvesteesMember_8b3c47c4-f1ee-433f-993a-e1bf0bee8d83" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeOrGroupOfInvesteesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeDomain_c1438bb2-9458-448e-b28f-4d8e49f86971" xlink:to="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeOrGroupOfInvesteesMember_8b3c47c4-f1ee-433f-993a-e1bf0bee8d83" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrent_4e0f2d03-0f22-4192-8aec-30033aa54e2d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AssetsCurrent"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_604185e0-eeb1-45d3-bc42-38570f198adc" xlink:to="loc_us-gaap_AssetsCurrent_4e0f2d03-0f22-4192-8aec-30033aa54e2d" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsNoncurrent_9dbad952-52b2-4f0c-af08-dcc323b5ff14" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AssetsNoncurrent"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_604185e0-eeb1-45d3-bc42-38570f198adc" xlink:to="loc_us-gaap_AssetsNoncurrent_9dbad952-52b2-4f0c-af08-dcc323b5ff14" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrent_c232e4c0-69df-4940-9dc2-76b9d0aabd69" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LiabilitiesCurrent"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_604185e0-eeb1-45d3-bc42-38570f198adc" xlink:to="loc_us-gaap_LiabilitiesCurrent_c232e4c0-69df-4940-9dc2-76b9d0aabd69" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesNoncurrent_faec00dc-8698-4e1a-b03e-702ec78033d5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LiabilitiesNoncurrent"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_604185e0-eeb1-45d3-bc42-38570f198adc" xlink:to="loc_us-gaap_LiabilitiesNoncurrent_faec00dc-8698-4e1a-b03e-702ec78033d5" xlink:type="arc" order="3"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/InvestmentsinAffiliatedCompaniesSummarizedIncomeStatementDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#InvestmentsinAffiliatedCompaniesSummarizedIncomeStatementDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/InvestmentsinAffiliatedCompaniesSummarizedIncomeStatementDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_eac31a33-1ffa-4f7c-98c4-8b84754ac018" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_b9fb77ec-1e0a-49db-9904-1a284d1149fb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_eac31a33-1ffa-4f7c-98c4-8b84754ac018" xlink:to="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_b9fb77ec-1e0a-49db-9904-1a284d1149fb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeAxis_e9c94c4c-b13a-47d9-bc51-3660535540f1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_b9fb77ec-1e0a-49db-9904-1a284d1149fb" xlink:to="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeAxis_e9c94c4c-b13a-47d9-bc51-3660535540f1" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeDomain_e9c94c4c-b13a-47d9-bc51-3660535540f1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeAxis_e9c94c4c-b13a-47d9-bc51-3660535540f1" xlink:to="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeDomain_e9c94c4c-b13a-47d9-bc51-3660535540f1_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeDomain_73b399ea-9257-4295-b7a8-e9e2739235e3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeAxis_e9c94c4c-b13a-47d9-bc51-3660535540f1" xlink:to="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeDomain_73b399ea-9257-4295-b7a8-e9e2739235e3" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeOrGroupOfInvesteesMember_4cadb321-5df1-465f-afe2-88f3ada26596" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeOrGroupOfInvesteesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeDomain_73b399ea-9257-4295-b7a8-e9e2739235e3" xlink:to="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeOrGroupOfInvesteesMember_4cadb321-5df1-465f-afe2-88f3ada26596" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_0a73014a-4de1-40e9-8656-c66ee690514e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_Revenues"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_eac31a33-1ffa-4f7c-98c4-8b84754ac018" xlink:to="loc_us-gaap_Revenues_0a73014a-4de1-40e9-8656-c66ee690514e" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_GrossProfit_ac35e730-bb6f-4b40-93c3-7690d8d17a8a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_GrossProfit"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_eac31a33-1ffa-4f7c-98c4-8b84754ac018" xlink:to="loc_us-gaap_GrossProfit_ac35e730-bb6f-4b40-93c3-7690d8d17a8a" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_60e94dfe-ab2b-4ec3-b0e6-1c7ff5f7e68a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OperatingIncomeLoss"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_eac31a33-1ffa-4f7c-98c4-8b84754ac018" xlink:to="loc_us-gaap_OperatingIncomeLoss_60e94dfe-ab2b-4ec3-b0e6-1c7ff5f7e68a" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_de0beeda-7b0f-45c1-91d4-dfe8c229c1de" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NetIncomeLoss"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_eac31a33-1ffa-4f7c-98c4-8b84754ac018" xlink:to="loc_us-gaap_NetIncomeLoss_de0beeda-7b0f-45c1-91d4-dfe8c229c1de" xlink:type="arc" order="3"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/PropertyPlantandEquipmentDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#PropertyPlantandEquipmentDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/PropertyPlantandEquipmentDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_3901fbd7-24c4-4bcf-8ad2-c36a0b4efebe" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_733be34c-d19c-4e60-afba-88fef46b588c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_3901fbd7-24c4-4bcf-8ad2-c36a0b4efebe" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_733be34c-d19c-4e60-afba-88fef46b588c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_c730308f-b75a-4fb7-abb8-59403b4dc9a8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_733be34c-d19c-4e60-afba-88fef46b588c" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_c730308f-b75a-4fb7-abb8-59403b4dc9a8" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_c730308f-b75a-4fb7-abb8-59403b4dc9a8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_c730308f-b75a-4fb7-abb8-59403b4dc9a8" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_c730308f-b75a-4fb7-abb8-59403b4dc9a8_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_1ec64e63-1657-4d60-b0dd-52eaa4bc6ddc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_c730308f-b75a-4fb7-abb8-59403b4dc9a8" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_1ec64e63-1657-4d60-b0dd-52eaa4bc6ddc" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LandMember_a7d58601-2384-4dbf-8390-ebb779deb7bf" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LandMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_1ec64e63-1657-4d60-b0dd-52eaa4bc6ddc" xlink:to="loc_us-gaap_LandMember_a7d58601-2384-4dbf-8390-ebb779deb7bf" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_BuildingAndBuildingImprovementsMember_e00a7b7a-5a07-4d4d-a069-1588210ed770" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BuildingAndBuildingImprovementsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_1ec64e63-1657-4d60-b0dd-52eaa4bc6ddc" xlink:to="loc_us-gaap_BuildingAndBuildingImprovementsMember_e00a7b7a-5a07-4d4d-a069-1588210ed770" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_MachineryAndEquipmentMember_f8317b52-961d-4d93-9353-0646b54aea00" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MachineryAndEquipmentMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_1ec64e63-1657-4d60-b0dd-52eaa4bc6ddc" xlink:to="loc_us-gaap_MachineryAndEquipmentMember_f8317b52-961d-4d93-9353-0646b54aea00" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConstructionInProgressMember_8c8c68e4-c9e9-43ab-a497-64453824dbb2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ConstructionInProgressMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_1ec64e63-1657-4d60-b0dd-52eaa4bc6ddc" xlink:to="loc_us-gaap_ConstructionInProgressMember_8c8c68e4-c9e9-43ab-a497-64453824dbb2" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_2d91dd45-9af4-4d2f-9b80-f1a11413f42a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentGross"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_3901fbd7-24c4-4bcf-8ad2-c36a0b4efebe" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_2d91dd45-9af4-4d2f-9b80-f1a11413f42a" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_0bbd8686-f5f9-4a60-aaf0-35329232c6ee" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_3901fbd7-24c4-4bcf-8ad2-c36a0b4efebe" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_0bbd8686-f5f9-4a60-aaf0-35329232c6ee" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_2816fd05-575c-4a05-94a2-c6e8fa5db245" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_3901fbd7-24c4-4bcf-8ad2-c36a0b4efebe" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_2816fd05-575c-4a05-94a2-c6e8fa5db245" xlink:type="arc" order="2"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/ReportableSegmentsSummaryFinancialInformationbyReportableSegmentDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#ReportableSegmentsSummaryFinancialInformationbyReportableSegmentDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/ReportableSegmentsSummaryFinancialInformationbyReportableSegmentDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingRevenueReconcilingItemLineItems_64ec02fa-2e36-4632-879e-ec995cec287e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentReportingRevenueReconcilingItemLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReconciliationOfRevenueFromSegmentsToConsolidatedTable_3387d6dd-2426-439a-a0ed-58fcf6a1517c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ReconciliationOfRevenueFromSegmentsToConsolidatedTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingRevenueReconcilingItemLineItems_64ec02fa-2e36-4632-879e-ec995cec287e" xlink:to="loc_us-gaap_ReconciliationOfRevenueFromSegmentsToConsolidatedTable_3387d6dd-2426-439a-a0ed-58fcf6a1517c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_c32d3371-68c2-4229-989c-475556928252" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementBusinessSegmentsAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReconciliationOfRevenueFromSegmentsToConsolidatedTable_3387d6dd-2426-439a-a0ed-58fcf6a1517c" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_c32d3371-68c2-4229-989c-475556928252" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_c32d3371-68c2-4229-989c-475556928252_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_c32d3371-68c2-4229-989c-475556928252" xlink:to="loc_us-gaap_SegmentDomain_c32d3371-68c2-4229-989c-475556928252_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_fafdeecc-be63-4f2d-bfcf-a5565d7aeb5c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_c32d3371-68c2-4229-989c-475556928252" xlink:to="loc_us-gaap_SegmentDomain_fafdeecc-be63-4f2d-bfcf-a5565d7aeb5c" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_EcoservicesMember_c246609a-a4e0-4cbc-81ae-e007b45682da" xlink:href="ecvt-20231231.xsd#ecvt_EcoservicesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_fafdeecc-be63-4f2d-bfcf-a5565d7aeb5c" xlink:to="loc_ecvt_EcoservicesMember_c246609a-a4e0-4cbc-81ae-e007b45682da" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_AdvancedMaterialsAndCatalystsMember_0fdc4db0-1141-4c85-a8db-bbc71b26d637" xlink:href="ecvt-20231231.xsd#ecvt_AdvancedMaterialsAndCatalystsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_fafdeecc-be63-4f2d-bfcf-a5565d7aeb5c" xlink:to="loc_ecvt_AdvancedMaterialsAndCatalystsMember_0fdc4db0-1141-4c85-a8db-bbc71b26d637" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_e3a09c26-9914-4835-826a-6b9bc421c61f" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReconciliationOfRevenueFromSegmentsToConsolidatedTable_3387d6dd-2426-439a-a0ed-58fcf6a1517c" xlink:to="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_e3a09c26-9914-4835-826a-6b9bc421c61f" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_e3a09c26-9914-4835-826a-6b9bc421c61f_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_EquityMethodInvesteeNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_e3a09c26-9914-4835-826a-6b9bc421c61f" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_e3a09c26-9914-4835-826a-6b9bc421c61f_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_462aade8-fcdf-4046-a06e-221767d4f602" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_EquityMethodInvesteeNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_e3a09c26-9914-4835-826a-6b9bc421c61f" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_462aade8-fcdf-4046-a06e-221767d4f602" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_ZeolystJointVentureMember_df435ffa-d8dd-401e-b648-4a3f54168186" xlink:href="ecvt-20231231.xsd#ecvt_ZeolystJointVentureMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_462aade8-fcdf-4046-a06e-221767d4f602" xlink:to="loc_ecvt_ZeolystJointVentureMember_df435ffa-d8dd-401e-b648-4a3f54168186" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_9e72981a-c0b6-4654-828a-c6ef439834d9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingRevenueReconcilingItemLineItems_64ec02fa-2e36-4632-879e-ec995cec287e" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_9e72981a-c0b6-4654-828a-c6ef439834d9" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_AdjustedEarningsBeforeInterestTaxesDepreciationAndAmortization_a21ba95c-30d8-4016-a909-4c2ec7ea06cb" xlink:href="ecvt-20231231.xsd#ecvt_AdjustedEarningsBeforeInterestTaxesDepreciationAndAmortization"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingRevenueReconcilingItemLineItems_64ec02fa-2e36-4632-879e-ec995cec287e" xlink:to="loc_ecvt_AdjustedEarningsBeforeInterestTaxesDepreciationAndAmortization_a21ba95c-30d8-4016-a909-4c2ec7ea06cb" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestments_2d0d307e-534e-4c03-a515-adcc31ac9ac9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeLossFromEquityMethodInvestments"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingRevenueReconcilingItemLineItems_64ec02fa-2e36-4632-879e-ec995cec287e" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestments_2d0d307e-534e-4c03-a515-adcc31ac9ac9" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_EquityMethodInvestmentsChargesRelatedtoPurchaseAccounting_a11a12a0-27e4-4ad1-86c9-e457792e89cb" xlink:href="ecvt-20231231.xsd#ecvt_EquityMethodInvestmentsChargesRelatedtoPurchaseAccounting"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingRevenueReconcilingItemLineItems_64ec02fa-2e36-4632-879e-ec995cec287e" xlink:to="loc_ecvt_EquityMethodInvestmentsChargesRelatedtoPurchaseAccounting_a11a12a0-27e4-4ad1-86c9-e457792e89cb" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_EquityMethodInvestmentAmortizationExpense_69dea693-4197-4e2b-a297-eb43e499bd3f" xlink:href="ecvt-20231231.xsd#ecvt_EquityMethodInvestmentAmortizationExpense"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingRevenueReconcilingItemLineItems_64ec02fa-2e36-4632-879e-ec995cec287e" xlink:to="loc_ecvt_EquityMethodInvestmentAmortizationExpense_69dea693-4197-4e2b-a297-eb43e499bd3f" xlink:type="arc" order="4"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/ReportableSegmentsReconciliationofNetIncometoAdjustedEBITDADetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#ReportableSegmentsReconciliationofNetIncometoAdjustedEBITDADetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/ReportableSegmentsReconciliationofNetIncometoAdjustedEBITDADetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_c3f660fd-9285-48ac-9cc0-dc1ff48bcd07" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentReportingInformationLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_748307c9-3aef-4e43-a255-3e280179f5ad" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_c3f660fd-9285-48ac-9cc0-dc1ff48bcd07" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_748307c9-3aef-4e43-a255-3e280179f5ad" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_9479238f-d482-4839-9f8b-a93c25582d6a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementBusinessSegmentsAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_748307c9-3aef-4e43-a255-3e280179f5ad" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_9479238f-d482-4839-9f8b-a93c25582d6a" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_9479238f-d482-4839-9f8b-a93c25582d6a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_9479238f-d482-4839-9f8b-a93c25582d6a" xlink:to="loc_us-gaap_SegmentDomain_9479238f-d482-4839-9f8b-a93c25582d6a_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_04fe5bae-fc25-45b6-a779-e2d1d93f7969" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_9479238f-d482-4839-9f8b-a93c25582d6a" xlink:to="loc_us-gaap_SegmentDomain_04fe5bae-fc25-45b6-a779-e2d1d93f7969" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateMember_fafbb891-51fb-4e4e-a80c-efa4ec93e2c4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CorporateMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_04fe5bae-fc25-45b6-a779-e2d1d93f7969" xlink:to="loc_us-gaap_CorporateMember_fafbb891-51fb-4e4e-a80c-efa4ec93e2c4" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_1f00ed44-d3a6-4829-8eef-b54af89031d2" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_748307c9-3aef-4e43-a255-3e280179f5ad" xlink:to="loc_srt_ConsolidationItemsAxis_1f00ed44-d3a6-4829-8eef-b54af89031d2" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_1f00ed44-d3a6-4829-8eef-b54af89031d2_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_1f00ed44-d3a6-4829-8eef-b54af89031d2" xlink:to="loc_srt_ConsolidationItemsDomain_1f00ed44-d3a6-4829-8eef-b54af89031d2_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_88652826-336d-4a77-ada4-b0a3b614aebb" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_1f00ed44-d3a6-4829-8eef-b54af89031d2" xlink:to="loc_srt_ConsolidationItemsDomain_88652826-336d-4a77-ada4-b0a3b614aebb" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_MaterialReconcilingItemsMember_8ed2014a-42d3-4737-ae62-1307e7105dc2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MaterialReconcilingItemsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_88652826-336d-4a77-ada4-b0a3b614aebb" xlink:to="loc_us-gaap_MaterialReconcilingItemsMember_8ed2014a-42d3-4737-ae62-1307e7105dc2" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_901066f2-51ed-477c-899e-d81c9ca5f35b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OperatingSegmentsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_88652826-336d-4a77-ada4-b0a3b614aebb" xlink:to="loc_us-gaap_OperatingSegmentsMember_901066f2-51ed-477c-899e-d81c9ca5f35b" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_d1a6e39a-098d-427f-860f-40965c5bca87" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_c3f660fd-9285-48ac-9cc0-dc1ff48bcd07" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_d1a6e39a-098d-427f-860f-40965c5bca87" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_d1a74bea-8880-4310-8137-8d19bf46966a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_InterestExpense"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_c3f660fd-9285-48ac-9cc0-dc1ff48bcd07" xlink:to="loc_us-gaap_InterestExpense_d1a74bea-8880-4310-8137-8d19bf46966a" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_41799096-41a9-4a95-8211-3a9fb433508a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DepreciationDepletionAndAmortization"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_c3f660fd-9285-48ac-9cc0-dc1ff48bcd07" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_41799096-41a9-4a95-8211-3a9fb433508a" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_UnallocatedCorporateExpenses_f1b245ff-c144-40e5-afb3-13d4aebe454a" xlink:href="ecvt-20231231.xsd#ecvt_UnallocatedCorporateExpenses"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_c3f660fd-9285-48ac-9cc0-dc1ff48bcd07" xlink:to="loc_ecvt_UnallocatedCorporateExpenses_f1b245ff-c144-40e5-afb3-13d4aebe454a" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_AdjustedEarningsBeforeInterestTaxesDepreciationAndAmortization_f50cf66f-7bdd-4782-a4da-121175c738cb" xlink:href="ecvt-20231231.xsd#ecvt_AdjustedEarningsBeforeInterestTaxesDepreciationAndAmortization"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_c3f660fd-9285-48ac-9cc0-dc1ff48bcd07" xlink:to="loc_ecvt_AdjustedEarningsBeforeInterestTaxesDepreciationAndAmortization_f50cf66f-7bdd-4782-a4da-121175c738cb" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_EquityMethodInvestmentAmortizationExpense_258fbf81-9dd5-468e-96d7-73962ec0f56a" xlink:href="ecvt-20231231.xsd#ecvt_EquityMethodInvestmentAmortizationExpense"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_c3f660fd-9285-48ac-9cc0-dc1ff48bcd07" xlink:to="loc_ecvt_EquityMethodInvestmentAmortizationExpense_258fbf81-9dd5-468e-96d7-73962ec0f56a" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_EquityMethodInvestmentsChargesRelatedtoPurchaseAccounting_73c2d059-30e6-42fe-af05-afadcfbd1cfb" xlink:href="ecvt-20231231.xsd#ecvt_EquityMethodInvestmentsChargesRelatedtoPurchaseAccounting"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_c3f660fd-9285-48ac-9cc0-dc1ff48bcd07" xlink:to="loc_ecvt_EquityMethodInvestmentsChargesRelatedtoPurchaseAccounting_73c2d059-30e6-42fe-af05-afadcfbd1cfb" xlink:type="arc" order="6"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_eb2d46e8-dc82-48e3-a460-aba82de4d4ba" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_c3f660fd-9285-48ac-9cc0-dc1ff48bcd07" xlink:to="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_eb2d46e8-dc82-48e3-a460-aba82de4d4ba" xlink:type="arc" order="7"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnDispositionOfAssets1_2a5b2441-a532-42fa-8701-8853fca32d99" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_GainLossOnDispositionOfAssets1"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_c3f660fd-9285-48ac-9cc0-dc1ff48bcd07" xlink:to="loc_us-gaap_GainLossOnDispositionOfAssets1_2a5b2441-a532-42fa-8701-8853fca32d99" xlink:type="arc" order="8"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCurrencyTransactionGainLossBeforeTax_4160481f-90a6-4d44-8e51-9c9fbf94d3ad" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ForeignCurrencyTransactionGainLossBeforeTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_c3f660fd-9285-48ac-9cc0-dc1ff48bcd07" xlink:to="loc_us-gaap_ForeignCurrencyTransactionGainLossBeforeTax_4160481f-90a6-4d44-8e51-9c9fbf94d3ad" xlink:type="arc" order="9"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryWriteDown_2501572c-0761-4966-8b3c-93ec7a106f07" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_InventoryWriteDown"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_c3f660fd-9285-48ac-9cc0-dc1ff48bcd07" xlink:to="loc_us-gaap_InventoryWriteDown_2501572c-0761-4966-8b3c-93ec7a106f07" xlink:type="arc" order="10"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_TransactionAndOtherRelatedCosts_329695c9-5227-4fcc-a7c1-6cd0aa697245" xlink:href="ecvt-20231231.xsd#ecvt_TransactionAndOtherRelatedCosts"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_c3f660fd-9285-48ac-9cc0-dc1ff48bcd07" xlink:to="loc_ecvt_TransactionAndOtherRelatedCosts_329695c9-5227-4fcc-a7c1-6cd0aa697245" xlink:type="arc" order="11"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_d82b7daf-7dd4-4e1b-a932-ced1736dc3e9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_c3f660fd-9285-48ac-9cc0-dc1ff48bcd07" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_d82b7daf-7dd4-4e1b-a932-ced1736dc3e9" xlink:type="arc" order="12"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_BusinessOptimizationIntegrationRestructuringAndOtherRelatedCosts_d47d6bda-0c95-4936-965b-b034453c3262" xlink:href="ecvt-20231231.xsd#ecvt_BusinessOptimizationIntegrationRestructuringAndOtherRelatedCosts"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_c3f660fd-9285-48ac-9cc0-dc1ff48bcd07" xlink:to="loc_ecvt_BusinessOptimizationIntegrationRestructuringAndOtherRelatedCosts_d47d6bda-0c95-4936-965b-b034453c3262" xlink:type="arc" order="13"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_82357200-12c9-4c9e-9afc-4747f364b082" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_c3f660fd-9285-48ac-9cc0-dc1ff48bcd07" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_82357200-12c9-4c9e-9afc-4747f364b082" xlink:type="arc" order="14"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/ReportableSegmentsCapitalExpendituresDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#ReportableSegmentsCapitalExpendituresDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/ReportableSegmentsCapitalExpendituresDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_ed16432e-d5e5-4ceb-9da4-d8c1e17506f3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_3d2e4521-06f6-49cf-9d53-df40dda4fdfa" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_ed16432e-d5e5-4ceb-9da4-d8c1e17506f3" xlink:to="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_3d2e4521-06f6-49cf-9d53-df40dda4fdfa" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_2491f22f-e37e-40da-a385-5749cd300bb9" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_3d2e4521-06f6-49cf-9d53-df40dda4fdfa" xlink:to="loc_srt_ConsolidationItemsAxis_2491f22f-e37e-40da-a385-5749cd300bb9" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_2491f22f-e37e-40da-a385-5749cd300bb9_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_2491f22f-e37e-40da-a385-5749cd300bb9" xlink:to="loc_srt_ConsolidationItemsDomain_2491f22f-e37e-40da-a385-5749cd300bb9_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_9f82dfbe-5ce8-46a8-a6fc-2b5ad04f441e" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_2491f22f-e37e-40da-a385-5749cd300bb9" xlink:to="loc_srt_ConsolidationItemsDomain_9f82dfbe-5ce8-46a8-a6fc-2b5ad04f441e" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_711f78c5-d05e-43ed-9b4c-8e101062c05b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OperatingSegmentsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_9f82dfbe-5ce8-46a8-a6fc-2b5ad04f441e" xlink:to="loc_us-gaap_OperatingSegmentsMember_711f78c5-d05e-43ed-9b4c-8e101062c05b" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_8d799b38-5a94-4c54-9dd8-f8f4da50295a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementBusinessSegmentsAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_3d2e4521-06f6-49cf-9d53-df40dda4fdfa" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_8d799b38-5a94-4c54-9dd8-f8f4da50295a" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_8d799b38-5a94-4c54-9dd8-f8f4da50295a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_8d799b38-5a94-4c54-9dd8-f8f4da50295a" xlink:to="loc_us-gaap_SegmentDomain_8d799b38-5a94-4c54-9dd8-f8f4da50295a_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_08b40ee1-d884-49d8-a474-eb278fa7e3ce" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_8d799b38-5a94-4c54-9dd8-f8f4da50295a" xlink:to="loc_us-gaap_SegmentDomain_08b40ee1-d884-49d8-a474-eb278fa7e3ce" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_EcoservicesMember_2f3f6e2a-c446-4b94-a821-445ecacf3749" xlink:href="ecvt-20231231.xsd#ecvt_EcoservicesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_08b40ee1-d884-49d8-a474-eb278fa7e3ce" xlink:to="loc_ecvt_EcoservicesMember_2f3f6e2a-c446-4b94-a821-445ecacf3749" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_AdvancedMaterialsAndCatalystsMember_dd26091d-b98c-4e0b-9b5e-e1e9e6d1741a" xlink:href="ecvt-20231231.xsd#ecvt_AdvancedMaterialsAndCatalystsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_08b40ee1-d884-49d8-a474-eb278fa7e3ce" xlink:to="loc_ecvt_AdvancedMaterialsAndCatalystsMember_dd26091d-b98c-4e0b-9b5e-e1e9e6d1741a" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateMember_fca7043f-b049-4cdf-a22f-676dc0338306" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CorporateMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_08b40ee1-d884-49d8-a474-eb278fa7e3ce" xlink:to="loc_us-gaap_CorporateMember_fca7043f-b049-4cdf-a22f-676dc0338306" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentExpenditureAdditionToLongLivedAssets_8241b22a-be32-4e88-9c3b-0811e3d99d85" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentExpenditureAdditionToLongLivedAssets"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_ed16432e-d5e5-4ceb-9da4-d8c1e17506f3" xlink:to="loc_us-gaap_SegmentExpenditureAdditionToLongLivedAssets_8241b22a-be32-4e88-9c3b-0811e3d99d85" xlink:type="arc" order="0"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/ReportableSegmentsSalesbyGeographyDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#ReportableSegmentsSalesbyGeographyDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/ReportableSegmentsSalesbyGeographyDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_34ed61ca-256b-48ec-95ba-1b8423b9997a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_34c75ee5-1481-45e1-a797-c8f50bba88d7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_34ed61ca-256b-48ec-95ba-1b8423b9997a" xlink:to="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_34c75ee5-1481-45e1-a797-c8f50bba88d7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_3dcc316e-64a1-4c84-9629-d4167b1444d8" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_StatementGeographicalAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_34c75ee5-1481-45e1-a797-c8f50bba88d7" xlink:to="loc_srt_StatementGeographicalAxis_3dcc316e-64a1-4c84-9629-d4167b1444d8" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_3dcc316e-64a1-4c84-9629-d4167b1444d8_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_SegmentGeographicalDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_3dcc316e-64a1-4c84-9629-d4167b1444d8" xlink:to="loc_srt_SegmentGeographicalDomain_3dcc316e-64a1-4c84-9629-d4167b1444d8_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_0563cc2d-157a-4d7d-b902-f8f411868a4e" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_SegmentGeographicalDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_3dcc316e-64a1-4c84-9629-d4167b1444d8" xlink:to="loc_srt_SegmentGeographicalDomain_0563cc2d-157a-4d7d-b902-f8f411868a4e" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_country_US_0304fc91-386a-4712-bc7e-33ed6b3b2e33" xlink:href="https://xbrl.sec.gov/country/2023/country-2023.xsd#country_US"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_0563cc2d-157a-4d7d-b902-f8f411868a4e" xlink:to="loc_country_US_0304fc91-386a-4712-bc7e-33ed6b3b2e33" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_OtherForeignCountriesMember_d30da67f-5628-4613-931f-e6ba83794eac" xlink:href="ecvt-20231231.xsd#ecvt_OtherForeignCountriesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_0563cc2d-157a-4d7d-b902-f8f411868a4e" xlink:to="loc_ecvt_OtherForeignCountriesMember_d30da67f-5628-4613-931f-e6ba83794eac" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_5dc5f0fa-8e49-46a9-a775-b1698a6a0274" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ProductOrServiceAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_34c75ee5-1481-45e1-a797-c8f50bba88d7" xlink:to="loc_srt_ProductOrServiceAxis_5dc5f0fa-8e49-46a9-a775-b1698a6a0274" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_5dc5f0fa-8e49-46a9-a775-b1698a6a0274_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ProductsAndServicesDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_5dc5f0fa-8e49-46a9-a775-b1698a6a0274" xlink:to="loc_srt_ProductsAndServicesDomain_5dc5f0fa-8e49-46a9-a775-b1698a6a0274_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_9a04f29e-a945-40a7-8b84-18985d61e4a4" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ProductsAndServicesDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_5dc5f0fa-8e49-46a9-a775-b1698a6a0274" xlink:to="loc_srt_ProductsAndServicesDomain_9a04f29e-a945-40a7-8b84-18985d61e4a4" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_ProductsMember_d37bc08a-9a7f-43f4-b81f-d18a9f9aa97d" xlink:href="ecvt-20231231.xsd#ecvt_ProductsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_9a04f29e-a945-40a7-8b84-18985d61e4a4" xlink:to="loc_ecvt_ProductsMember_d37bc08a-9a7f-43f4-b81f-d18a9f9aa97d" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_2eee4eeb-065a-4075-af3b-b2ff93b12619" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_34c75ee5-1481-45e1-a797-c8f50bba88d7" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_2eee4eeb-065a-4075-af3b-b2ff93b12619" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_2eee4eeb-065a-4075-af3b-b2ff93b12619_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_2eee4eeb-065a-4075-af3b-b2ff93b12619" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_2eee4eeb-065a-4075-af3b-b2ff93b12619_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_4ac5953e-89bb-42fa-ba88-7bc53b8937ac" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_2eee4eeb-065a-4075-af3b-b2ff93b12619" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_4ac5953e-89bb-42fa-ba88-7bc53b8937ac" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SalesRevenueNetMember_9160eca1-4852-414a-b1e6-8b5dd94c692e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SalesRevenueNetMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_4ac5953e-89bb-42fa-ba88-7bc53b8937ac" xlink:to="loc_us-gaap_SalesRevenueNetMember_9160eca1-4852-414a-b1e6-8b5dd94c692e" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_6771a0d8-b3e7-4d11-b187-23d763c2b23b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_34c75ee5-1481-45e1-a797-c8f50bba88d7" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_6771a0d8-b3e7-4d11-b187-23d763c2b23b" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_6771a0d8-b3e7-4d11-b187-23d763c2b23b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ConcentrationRiskTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_6771a0d8-b3e7-4d11-b187-23d763c2b23b" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_6771a0d8-b3e7-4d11-b187-23d763c2b23b_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_bca0d144-5522-48d6-8d76-54969abfca1f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ConcentrationRiskTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_6771a0d8-b3e7-4d11-b187-23d763c2b23b" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_bca0d144-5522-48d6-8d76-54969abfca1f" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerConcentrationRiskMember_f0660415-bff2-4173-9f10-fba58089afc8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CustomerConcentrationRiskMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_bca0d144-5522-48d6-8d76-54969abfca1f" xlink:to="loc_us-gaap_CustomerConcentrationRiskMember_f0660415-bff2-4173-9f10-fba58089afc8" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_07c831b8-0965-40e6-bbae-2f2c351b5d87" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_34ed61ca-256b-48ec-95ba-1b8423b9997a" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_07c831b8-0965-40e6-bbae-2f2c351b5d87" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_ef7bc8ec-4373-4f90-aa24-d0a0cfb65f92" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ConcentrationRiskPercentage1"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_34ed61ca-256b-48ec-95ba-1b8423b9997a" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_ef7bc8ec-4373-4f90-aa24-d0a0cfb65f92" xlink:type="arc" order="1"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/ReportableSegmentsAssetsbyGeographyDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#ReportableSegmentsAssetsbyGeographyDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/ReportableSegmentsAssetsbyGeographyDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_d6264e0d-ac7b-4704-bb0f-da0ecc21fa68" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_7f3d34d5-a6c0-4fb5-92a5-db9488f185c5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_d6264e0d-ac7b-4704-bb0f-da0ecc21fa68" xlink:to="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_7f3d34d5-a6c0-4fb5-92a5-db9488f185c5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_20b16b89-9e0b-46e8-8d6b-0a539a3a60cd" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_StatementGeographicalAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_7f3d34d5-a6c0-4fb5-92a5-db9488f185c5" xlink:to="loc_srt_StatementGeographicalAxis_20b16b89-9e0b-46e8-8d6b-0a539a3a60cd" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_20b16b89-9e0b-46e8-8d6b-0a539a3a60cd_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_SegmentGeographicalDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_20b16b89-9e0b-46e8-8d6b-0a539a3a60cd" xlink:to="loc_srt_SegmentGeographicalDomain_20b16b89-9e0b-46e8-8d6b-0a539a3a60cd_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_7913dc8a-f1dc-4757-a02a-b989fcbb9ccf" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_SegmentGeographicalDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_20b16b89-9e0b-46e8-8d6b-0a539a3a60cd" xlink:to="loc_srt_SegmentGeographicalDomain_7913dc8a-f1dc-4757-a02a-b989fcbb9ccf" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_country_US_12211cab-bdc0-4e1e-99b6-75448fbcf781" xlink:href="https://xbrl.sec.gov/country/2023/country-2023.xsd#country_US"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_7913dc8a-f1dc-4757-a02a-b989fcbb9ccf" xlink:to="loc_country_US_12211cab-bdc0-4e1e-99b6-75448fbcf781" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_OtherForeignCountriesMember_bbce440a-034a-43bf-9980-8f4466917d22" xlink:href="ecvt-20231231.xsd#ecvt_OtherForeignCountriesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_7913dc8a-f1dc-4757-a02a-b989fcbb9ccf" xlink:to="loc_ecvt_OtherForeignCountriesMember_bbce440a-034a-43bf-9980-8f4466917d22" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncurrentAssets_8cd15d82-8159-4a1f-8ba2-1e03085584ab" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NoncurrentAssets"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_d6264e0d-ac7b-4704-bb0f-da0ecc21fa68" xlink:to="loc_us-gaap_NoncurrentAssets_8cd15d82-8159-4a1f-8ba2-1e03085584ab" xlink:type="arc" order="0"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/GoodwillandOtherIntangibleAssetsGoodwillRollforwardDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#GoodwillandOtherIntangibleAssetsGoodwillRollforwardDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/GoodwillandOtherIntangibleAssetsGoodwillRollforwardDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillLineItems_77f92287-d8a7-4fc9-831a-dc345600c625" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_GoodwillLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfGoodwillTable_7614dab4-58f9-412c-a942-496ce99c7e19" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfGoodwillTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_GoodwillLineItems_77f92287-d8a7-4fc9-831a-dc345600c625" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_7614dab4-58f9-412c-a942-496ce99c7e19" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_43010eea-04a7-4f5b-a15a-5d06e1bd59ff" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementBusinessSegmentsAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_7614dab4-58f9-412c-a942-496ce99c7e19" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_43010eea-04a7-4f5b-a15a-5d06e1bd59ff" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_43010eea-04a7-4f5b-a15a-5d06e1bd59ff_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_43010eea-04a7-4f5b-a15a-5d06e1bd59ff" xlink:to="loc_us-gaap_SegmentDomain_43010eea-04a7-4f5b-a15a-5d06e1bd59ff_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_501cf3e6-9dc3-4a3e-87ab-26ca2e640c1a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_43010eea-04a7-4f5b-a15a-5d06e1bd59ff" xlink:to="loc_us-gaap_SegmentDomain_501cf3e6-9dc3-4a3e-87ab-26ca2e640c1a" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_EcoservicesMember_86e3f1ca-bc5f-4e0a-a81c-57286bd0f9e8" xlink:href="ecvt-20231231.xsd#ecvt_EcoservicesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_501cf3e6-9dc3-4a3e-87ab-26ca2e640c1a" xlink:to="loc_ecvt_EcoservicesMember_86e3f1ca-bc5f-4e0a-a81c-57286bd0f9e8" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_AdvancedMaterialsAndCatalystsMember_2eec8ecc-3175-45d7-9b95-ec887b63ccac" xlink:href="ecvt-20231231.xsd#ecvt_AdvancedMaterialsAndCatalystsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_501cf3e6-9dc3-4a3e-87ab-26ca2e640c1a" xlink:to="loc_ecvt_AdvancedMaterialsAndCatalystsMember_2eec8ecc-3175-45d7-9b95-ec887b63ccac" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillRollForward_167720b7-aa63-465e-8dfa-4006e370961e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_GoodwillRollForward"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_77f92287-d8a7-4fc9-831a-dc345600c625" xlink:to="loc_us-gaap_GoodwillRollForward_167720b7-aa63-465e-8dfa-4006e370961e" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_8915374b-95e5-4747-80bf-90e1f0c99d51" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_Goodwill"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_167720b7-aa63-465e-8dfa-4006e370961e" xlink:to="loc_us-gaap_Goodwill_8915374b-95e5-4747-80bf-90e1f0c99d51" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillPurchaseAccountingAdjustments_d92b26bd-befb-4862-958d-b059de6f4363" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_GoodwillPurchaseAccountingAdjustments"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_167720b7-aa63-465e-8dfa-4006e370961e" xlink:to="loc_us-gaap_GoodwillPurchaseAccountingAdjustments_d92b26bd-befb-4862-958d-b059de6f4363" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillForeignCurrencyTranslationGainLoss_2af49770-57a2-4144-b3d0-7d7b4b4d8781" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_GoodwillForeignCurrencyTranslationGainLoss"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_167720b7-aa63-465e-8dfa-4006e370961e" xlink:to="loc_us-gaap_GoodwillForeignCurrencyTranslationGainLoss_2af49770-57a2-4144-b3d0-7d7b4b4d8781" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_c8ee8e69-c2e0-40ef-b6ac-2b132d3e9ff8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_Goodwill"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/GoodwillandOtherIntangibleAssetsNarrativesDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#GoodwillandOtherIntangibleAssetsNarrativesDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/GoodwillandOtherIntangibleAssetsNarrativesDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillLineItems_92c004b3-0622-4ed5-8e17-bd44adb4c2ae" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_GoodwillLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfGoodwillTable_d86183bd-9677-451c-8f3b-8051b2f14ec5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfGoodwillTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_GoodwillLineItems_92c004b3-0622-4ed5-8e17-bd44adb4c2ae" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_d86183bd-9677-451c-8f3b-8051b2f14ec5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_ae3e0983-cdd3-4748-9c36-7fe6d6abd4d7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeStatementLocationAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_d86183bd-9677-451c-8f3b-8051b2f14ec5" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_ae3e0983-cdd3-4748-9c36-7fe6d6abd4d7" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_ae3e0983-cdd3-4748-9c36-7fe6d6abd4d7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeStatementLocationDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_ae3e0983-cdd3-4748-9c36-7fe6d6abd4d7" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_ae3e0983-cdd3-4748-9c36-7fe6d6abd4d7_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_37dafd3a-47c1-4acd-8a7e-b3caef72aa35" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeStatementLocationDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_ae3e0983-cdd3-4748-9c36-7fe6d6abd4d7" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_37dafd3a-47c1-4acd-8a7e-b3caef72aa35" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfSalesMember_db81cc8d-f5c9-42e1-bc6b-f669397e0cdd" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CostOfSalesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_37dafd3a-47c1-4acd-8a7e-b3caef72aa35" xlink:to="loc_us-gaap_CostOfSalesMember_db81cc8d-f5c9-42e1-bc6b-f669397e0cdd" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherOperatingIncomeExpenseMember_4b807757-98ce-451f-9616-eacfc6025ff7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherOperatingIncomeExpenseMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_37dafd3a-47c1-4acd-8a7e-b3caef72aa35" xlink:to="loc_us-gaap_OtherOperatingIncomeExpenseMember_4b807757-98ce-451f-9616-eacfc6025ff7" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_e7168c99-09a7-499b-b2be-80eb2c848164" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_d86183bd-9677-451c-8f3b-8051b2f14ec5" xlink:to="loc_srt_RangeAxis_e7168c99-09a7-499b-b2be-80eb2c848164" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_e7168c99-09a7-499b-b2be-80eb2c848164_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_e7168c99-09a7-499b-b2be-80eb2c848164" xlink:to="loc_srt_RangeMember_e7168c99-09a7-499b-b2be-80eb2c848164_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_f4370bac-91b4-4afd-acfe-e5697f5757f7" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_e7168c99-09a7-499b-b2be-80eb2c848164" xlink:to="loc_srt_RangeMember_f4370bac-91b4-4afd-acfe-e5697f5757f7" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_705bdccc-d442-415c-a972-7b9d5b8f93ce" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MinimumMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_f4370bac-91b4-4afd-acfe-e5697f5757f7" xlink:to="loc_srt_MinimumMember_705bdccc-d442-415c-a972-7b9d5b8f93ce" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_3ada7820-98fb-4216-a07b-ee77dba6df3e" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MaximumMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_f4370bac-91b4-4afd-acfe-e5697f5757f7" xlink:to="loc_srt_MaximumMember_3ada7820-98fb-4216-a07b-ee77dba6df3e" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_73a3cd9f-bd5f-4b80-8b82-eb6172053ae3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_d86183bd-9677-451c-8f3b-8051b2f14ec5" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_73a3cd9f-bd5f-4b80-8b82-eb6172053ae3" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_73a3cd9f-bd5f-4b80-8b82-eb6172053ae3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_73a3cd9f-bd5f-4b80-8b82-eb6172053ae3" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_73a3cd9f-bd5f-4b80-8b82-eb6172053ae3_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_8f12b8ad-fb32-481c-88b7-ae6aa695badc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_73a3cd9f-bd5f-4b80-8b82-eb6172053ae3" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_8f12b8ad-fb32-481c-88b7-ae6aa695badc" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntellectualPropertyMember_659b40ad-e18c-47a7-84a6-31811918cda5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IntellectualPropertyMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_8f12b8ad-fb32-481c-88b7-ae6aa695badc" xlink:to="loc_us-gaap_IntellectualPropertyMember_659b40ad-e18c-47a7-84a6-31811918cda5" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_529773ac-44f9-495a-b528-e5685d7e45f1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CustomerRelationshipsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_8f12b8ad-fb32-481c-88b7-ae6aa695badc" xlink:to="loc_us-gaap_CustomerRelationshipsMember_529773ac-44f9-495a-b528-e5685d7e45f1" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_9b6bc54a-fb30-4844-b6d9-f751c966bee1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NoncompeteAgreementsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_8f12b8ad-fb32-481c-88b7-ae6aa695badc" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_9b6bc54a-fb30-4844-b6d9-f751c966bee1" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_TrademarksMember_109ebc95-7233-4f6f-9984-3770170a356a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TrademarksMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_8f12b8ad-fb32-481c-88b7-ae6aa695badc" xlink:to="loc_us-gaap_TrademarksMember_109ebc95-7233-4f6f-9984-3770170a356a" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_023887a3-3f18-4b3e-a63d-1bd55b3e7d2e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TradeNamesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_8f12b8ad-fb32-481c-88b7-ae6aa695badc" xlink:to="loc_us-gaap_TradeNamesMember_023887a3-3f18-4b3e-a63d-1bd55b3e7d2e" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfReportingUnits_c7942648-67c9-4e83-9583-52ddf414bd9f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NumberOfReportingUnits"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_92c004b3-0622-4ed5-8e17-bd44adb4c2ae" xlink:to="loc_us-gaap_NumberOfReportingUnits_c7942648-67c9-4e83-9583-52ddf414bd9f" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfOperatingSegments_48824a57-5ca1-4bde-ac35-547b79b8793a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NumberOfOperatingSegments"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_92c004b3-0622-4ed5-8e17-bd44adb4c2ae" xlink:to="loc_us-gaap_NumberOfOperatingSegments_48824a57-5ca1-4bde-ac35-547b79b8793a" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_745be29c-d1b5-43fa-9486-47e18d515434" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_92c004b3-0622-4ed5-8e17-bd44adb4c2ae" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_745be29c-d1b5-43fa-9486-47e18d515434" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_4497e6f7-4b29-40dc-91fd-095cf1a69ec2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AmortizationOfIntangibleAssets"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_92c004b3-0622-4ed5-8e17-bd44adb4c2ae" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_4497e6f7-4b29-40dc-91fd-095cf1a69ec2" xlink:type="arc" order="3"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/GoodwillandOtherIntangibleAssetsScheduleofIntangibleAssetsDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#GoodwillandOtherIntangibleAssetsScheduleofIntangibleAssetsDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/GoodwillandOtherIntangibleAssetsScheduleofIntangibleAssetsDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_38db0374-7489-4ed5-a230-914bf24fd84f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_923ec5f6-f44a-4dba-bbe4-43ae00c7139d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_38db0374-7489-4ed5-a230-914bf24fd84f" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_923ec5f6-f44a-4dba-bbe4-43ae00c7139d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_ff78adc6-7bec-4356-9379-85bb01a39de0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_923ec5f6-f44a-4dba-bbe4-43ae00c7139d" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_ff78adc6-7bec-4356-9379-85bb01a39de0" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_ff78adc6-7bec-4356-9379-85bb01a39de0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_ff78adc6-7bec-4356-9379-85bb01a39de0" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_ff78adc6-7bec-4356-9379-85bb01a39de0_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_d4761a05-76d4-462d-beb9-5dd8e8651347" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_ff78adc6-7bec-4356-9379-85bb01a39de0" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_d4761a05-76d4-462d-beb9-5dd8e8651347" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMember_64517308-2344-4669-a05f-9a1eb34b4f17" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_d4761a05-76d4-462d-beb9-5dd8e8651347" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMember_64517308-2344-4669-a05f-9a1eb34b4f17" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntellectualPropertyMember_ebe30a6e-7e29-4c20-a0d8-b975bd7c3fdf" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IntellectualPropertyMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMember_64517308-2344-4669-a05f-9a1eb34b4f17" xlink:to="loc_us-gaap_IntellectualPropertyMember_ebe30a6e-7e29-4c20-a0d8-b975bd7c3fdf" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_24461a09-86cd-458b-bf19-182c90a1afab" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CustomerRelationshipsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMember_64517308-2344-4669-a05f-9a1eb34b4f17" xlink:to="loc_us-gaap_CustomerRelationshipsMember_24461a09-86cd-458b-bf19-182c90a1afab" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_d7b7a041-b03f-44e3-a6db-e4bb2793b01c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NoncompeteAgreementsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMember_64517308-2344-4669-a05f-9a1eb34b4f17" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_d7b7a041-b03f-44e3-a6db-e4bb2793b01c" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_TrademarksMember_ec0a0739-18eb-4ce1-b50b-6cb56ce34b23" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TrademarksMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMember_64517308-2344-4669-a05f-9a1eb34b4f17" xlink:to="loc_us-gaap_TrademarksMember_ec0a0739-18eb-4ce1-b50b-6cb56ce34b23" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_1637460d-efdd-433d-9367-05d2581c1607" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TradeNamesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMember_64517308-2344-4669-a05f-9a1eb34b4f17" xlink:to="loc_us-gaap_TradeNamesMember_1637460d-efdd-433d-9367-05d2581c1607" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_113325df-7e72-4c93-b7e5-1200e2bb2ab6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_923ec5f6-f44a-4dba-bbe4-43ae00c7139d" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_113325df-7e72-4c93-b7e5-1200e2bb2ab6" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_113325df-7e72-4c93-b7e5-1200e2bb2ab6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_113325df-7e72-4c93-b7e5-1200e2bb2ab6" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_113325df-7e72-4c93-b7e5-1200e2bb2ab6_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_8dc84785-feb4-43bf-a058-e940f46a32e6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_113325df-7e72-4c93-b7e5-1200e2bb2ab6" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_8dc84785-feb4-43bf-a058-e940f46a32e6" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_b304e27f-270f-42de-b728-4b84cc037fc9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TradeNamesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_8dc84785-feb4-43bf-a058-e940f46a32e6" xlink:to="loc_us-gaap_TradeNamesMember_b304e27f-270f-42de-b728-4b84cc037fc9" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_InProcessResearchAndDevelopmentMember_74821df7-bf3f-476e-a242-9a919d228716" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_InProcessResearchAndDevelopmentMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_8dc84785-feb4-43bf-a058-e940f46a32e6" xlink:to="loc_us-gaap_InProcessResearchAndDevelopmentMember_74821df7-bf3f-476e-a242-9a919d228716" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_c19c890c-7f71-43eb-8b10-0855981433bd" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_923ec5f6-f44a-4dba-bbe4-43ae00c7139d" xlink:to="loc_srt_RangeAxis_c19c890c-7f71-43eb-8b10-0855981433bd" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_c19c890c-7f71-43eb-8b10-0855981433bd_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_c19c890c-7f71-43eb-8b10-0855981433bd" xlink:to="loc_srt_RangeMember_c19c890c-7f71-43eb-8b10-0855981433bd_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_93fa9533-f8b6-41f9-946a-f5b2f02b4851" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_c19c890c-7f71-43eb-8b10-0855981433bd" xlink:to="loc_srt_RangeMember_93fa9533-f8b6-41f9-946a-f5b2f02b4851" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_8cfbe9db-78d7-4118-835e-6b459c1aace8" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MinimumMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_93fa9533-f8b6-41f9-946a-f5b2f02b4851" xlink:to="loc_srt_MinimumMember_8cfbe9db-78d7-4118-835e-6b459c1aace8" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_5f8a2109-e381-4c2a-99e6-64e3d61758c9" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MaximumMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_93fa9533-f8b6-41f9-946a-f5b2f02b4851" xlink:to="loc_srt_MaximumMember_5f8a2109-e381-4c2a-99e6-64e3d61758c9" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntangibleAssetsGrossExcludingGoodwill_de5e834b-f49e-4893-b153-718d90db7080" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IntangibleAssetsGrossExcludingGoodwill"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_38db0374-7489-4ed5-a230-914bf24fd84f" xlink:to="loc_us-gaap_IntangibleAssetsGrossExcludingGoodwill_de5e834b-f49e-4893-b153-718d90db7080" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_67c8fbbf-07f4-49ac-b45a-1c52eec67e8a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_38db0374-7489-4ed5-a230-914bf24fd84f" xlink:to="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_67c8fbbf-07f4-49ac-b45a-1c52eec67e8a" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_ddfb7f7a-760e-4c1c-9ec2-9004f772c900" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsNetAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_38db0374-7489-4ed5-a230-914bf24fd84f" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_ddfb7f7a-760e-4c1c-9ec2-9004f772c900" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_654af200-3320-45b1-8c24-d97c1b288a3a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_ddfb7f7a-760e-4c1c-9ec2-9004f772c900" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_654af200-3320-45b1-8c24-d97c1b288a3a" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_ff9f1d90-6bd4-4e92-9a1a-5926c0719322" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_ddfb7f7a-760e-4c1c-9ec2-9004f772c900" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_ff9f1d90-6bd4-4e92-9a1a-5926c0719322" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_e3a7aecf-dd81-456b-bfc7-43d8664010fd" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_ddfb7f7a-760e-4c1c-9ec2-9004f772c900" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_e3a7aecf-dd81-456b-bfc7-43d8664010fd" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_a1ebab0f-4615-48bd-9e82-f50115a8e796" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_ddfb7f7a-760e-4c1c-9ec2-9004f772c900" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_a1ebab0f-4615-48bd-9e82-f50115a8e796" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_78b59771-8544-4d8e-8ef2-37d9f7f61ca8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AmortizationOfIntangibleAssets"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_ddfb7f7a-760e-4c1c-9ec2-9004f772c900" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_78b59771-8544-4d8e-8ef2-37d9f7f61ca8" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwillAbstract_393fac22-5325-4afd-94cd-7462dd21bdb6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwillAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_38db0374-7489-4ed5-a230-914bf24fd84f" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwillAbstract_393fac22-5325-4afd-94cd-7462dd21bdb6" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_IndefiniteLivedIntangibleAssetsExcludingGoodwillGross_230b9f50-4af3-4133-99f3-be9b642b0ded" xlink:href="ecvt-20231231.xsd#ecvt_IndefiniteLivedIntangibleAssetsExcludingGoodwillGross"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwillAbstract_393fac22-5325-4afd-94cd-7462dd21bdb6" xlink:to="loc_ecvt_IndefiniteLivedIntangibleAssetsExcludingGoodwillGross_230b9f50-4af3-4133-99f3-be9b642b0ded" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_IndefiniteLivedIntangibleAssetsExcludingGoodwillNet_1b429d69-eab6-438f-9362-435e99545a0d" xlink:href="ecvt-20231231.xsd#ecvt_IndefiniteLivedIntangibleAssetsExcludingGoodwillNet"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwillAbstract_393fac22-5325-4afd-94cd-7462dd21bdb6" xlink:to="loc_ecvt_IndefiniteLivedIntangibleAssetsExcludingGoodwillNet_1b429d69-eab6-438f-9362-435e99545a0d" xlink:type="arc" order="2"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/LongtermDebtDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#LongtermDebtDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/LongtermDebtDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_94142aee-5659-4d82-88d6-2e63d8c80719" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_7e9dd986-a3cc-4ad8-ae8c-d30ef681ffce" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_94142aee-5659-4d82-88d6-2e63d8c80719" xlink:to="loc_us-gaap_DebtInstrumentTable_7e9dd986-a3cc-4ad8-ae8c-d30ef681ffce" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_bd223f30-d0b2-4a21-8c37-dad25de14df0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongtermDebtTypeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_7e9dd986-a3cc-4ad8-ae8c-d30ef681ffce" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_bd223f30-d0b2-4a21-8c37-dad25de14df0" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_bd223f30-d0b2-4a21-8c37-dad25de14df0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongtermDebtTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_bd223f30-d0b2-4a21-8c37-dad25de14df0" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_bd223f30-d0b2-4a21-8c37-dad25de14df0_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_3159462e-2718-48fb-bccb-eff183f0de4d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongtermDebtTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_bd223f30-d0b2-4a21-8c37-dad25de14df0" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_3159462e-2718-48fb-bccb-eff183f0de4d" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_MediumTermNotesMember_6d9d58fb-ac6d-451f-843c-3515b6d81c3b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MediumTermNotesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_3159462e-2718-48fb-bccb-eff183f0de4d" xlink:to="loc_us-gaap_MediumTermNotesMember_6d9d58fb-ac6d-451f-843c-3515b6d81c3b" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_c2ef7363-be6c-4dc0-bab3-e7d794900357" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LineOfCreditMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_3159462e-2718-48fb-bccb-eff183f0de4d" xlink:to="loc_us-gaap_LineOfCreditMember_c2ef7363-be6c-4dc0-bab3-e7d794900357" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_f8c47661-8541-41d1-b958-c9e6d064921c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_7e9dd986-a3cc-4ad8-ae8c-d30ef681ffce" xlink:to="loc_us-gaap_DebtInstrumentAxis_f8c47661-8541-41d1-b958-c9e6d064921c" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_f8c47661-8541-41d1-b958-c9e6d064921c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_f8c47661-8541-41d1-b958-c9e6d064921c" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_f8c47661-8541-41d1-b958-c9e6d064921c_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_32501b38-6b82-4bf9-9544-1c86d55dffc0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_f8c47661-8541-41d1-b958-c9e6d064921c" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_32501b38-6b82-4bf9-9544-1c86d55dffc0" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_A2021TermLoanFacilityMember_a0de43b6-6087-46ef-a55a-5e9f25d6a186" xlink:href="ecvt-20231231.xsd#ecvt_A2021TermLoanFacilityMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_32501b38-6b82-4bf9-9544-1c86d55dffc0" xlink:to="loc_ecvt_A2021TermLoanFacilityMember_a0de43b6-6087-46ef-a55a-5e9f25d6a186" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_365ff814-c145-4937-b986-b4ed9cef46b4" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_7e9dd986-a3cc-4ad8-ae8c-d30ef681ffce" xlink:to="loc_srt_RangeAxis_365ff814-c145-4937-b986-b4ed9cef46b4" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_365ff814-c145-4937-b986-b4ed9cef46b4_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_365ff814-c145-4937-b986-b4ed9cef46b4" xlink:to="loc_srt_RangeMember_365ff814-c145-4937-b986-b4ed9cef46b4_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_0500f12f-5d6b-445d-aa92-ff289deac1ef" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_365ff814-c145-4937-b986-b4ed9cef46b4" xlink:to="loc_srt_RangeMember_0500f12f-5d6b-445d-aa92-ff289deac1ef" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_9e106809-0c73-4793-9065-681376b5f884" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MinimumMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_0500f12f-5d6b-445d-aa92-ff289deac1ef" xlink:to="loc_srt_MinimumMember_9e106809-0c73-4793-9065-681376b5f884" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_06f3e3cd-0aee-4e1f-b0cf-864697855832" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_VariableRateAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_7e9dd986-a3cc-4ad8-ae8c-d30ef681ffce" xlink:to="loc_us-gaap_VariableRateAxis_06f3e3cd-0aee-4e1f-b0cf-864697855832" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_06f3e3cd-0aee-4e1f-b0cf-864697855832_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_VariableRateDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_06f3e3cd-0aee-4e1f-b0cf-864697855832" xlink:to="loc_us-gaap_VariableRateDomain_06f3e3cd-0aee-4e1f-b0cf-864697855832_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_3414e17d-5efc-4239-bba6-72aa710e5c19" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_VariableRateDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_06f3e3cd-0aee-4e1f-b0cf-864697855832" xlink:to="loc_us-gaap_VariableRateDomain_3414e17d-5efc-4239-bba6-72aa710e5c19" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_SecuredOvernightFinancingRateMember_c36a179f-ee2e-4194-849e-654898276883" xlink:href="ecvt-20231231.xsd#ecvt_SecuredOvernightFinancingRateMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_3414e17d-5efc-4239-bba6-72aa710e5c19" xlink:to="loc_ecvt_SecuredOvernightFinancingRateMember_c36a179f-ee2e-4194-849e-654898276883" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_74563e2f-9a38-46e9-a02c-413c7d00a2ed" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentCarryingAmount"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_94142aee-5659-4d82-88d6-2e63d8c80719" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_74563e2f-9a38-46e9-a02c-413c7d00a2ed" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscount_39179dd9-3a92-4989-959a-ab63a239c958" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentUnamortizedDiscount"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_94142aee-5659-4d82-88d6-2e63d8c80719" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscount_39179dd9-3a92-4989-959a-ab63a239c958" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_99d13840-866f-4fdc-8b1b-18eefa6494a9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DeferredFinanceCostsNet"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_94142aee-5659-4d82-88d6-2e63d8c80719" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_99d13840-866f-4fdc-8b1b-18eefa6494a9" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_7521927e-147a-431d-80e2-5e5bd6ca4f70" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongTermDebt"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_94142aee-5659-4d82-88d6-2e63d8c80719" xlink:to="loc_us-gaap_LongTermDebt_7521927e-147a-431d-80e2-5e5bd6ca4f70" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtCurrent_04c30eca-8576-41d5-a208-3f68b488c474" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongTermDebtCurrent"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_94142aee-5659-4d82-88d6-2e63d8c80719" xlink:to="loc_us-gaap_LongTermDebtCurrent_04c30eca-8576-41d5-a208-3f68b488c474" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_01b7fc5a-9b12-425f-92bd-46b79e8025f5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongTermDebtNoncurrent"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_94142aee-5659-4d82-88d6-2e63d8c80719" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_01b7fc5a-9b12-425f-92bd-46b79e8025f5" xlink:type="arc" order="5"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/LongtermDebtABLFacilityDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#LongtermDebtABLFacilityDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/LongtermDebtABLFacilityDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_5cedb98c-afca-4f9a-a88d-f91a23e135f5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_fc0e3ffc-6a20-43c9-82d0-ae38e92a05e0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5cedb98c-afca-4f9a-a88d-f91a23e135f5" xlink:to="loc_us-gaap_DebtInstrumentTable_fc0e3ffc-6a20-43c9-82d0-ae38e92a05e0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_CurrencyAxis_b70a2f19-527b-44db-bcae-67a5491644c6" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_CurrencyAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_fc0e3ffc-6a20-43c9-82d0-ae38e92a05e0" xlink:to="loc_srt_CurrencyAxis_b70a2f19-527b-44db-bcae-67a5491644c6" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_currency_AllCurrenciesDomain_b70a2f19-527b-44db-bcae-67a5491644c6_default" xlink:href="https://xbrl.sec.gov/currency/2023/currency-2023.xsd#currency_AllCurrenciesDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CurrencyAxis_b70a2f19-527b-44db-bcae-67a5491644c6" xlink:to="loc_currency_AllCurrenciesDomain_b70a2f19-527b-44db-bcae-67a5491644c6_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_currency_AllCurrenciesDomain_e1101e2f-839e-44e3-97ed-98c3252403d5" xlink:href="https://xbrl.sec.gov/currency/2023/currency-2023.xsd#currency_AllCurrenciesDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CurrencyAxis_b70a2f19-527b-44db-bcae-67a5491644c6" xlink:to="loc_currency_AllCurrenciesDomain_e1101e2f-839e-44e3-97ed-98c3252403d5" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_currency_USD_ab7edcf6-8360-4798-9ce3-6cadbdf62ea0" xlink:href="https://xbrl.sec.gov/currency/2023/currency-2023.xsd#currency_USD"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_currency_AllCurrenciesDomain_e1101e2f-839e-44e3-97ed-98c3252403d5" xlink:to="loc_currency_USD_ab7edcf6-8360-4798-9ce3-6cadbdf62ea0" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_currency_EUR_c9f28bd8-90dd-4457-adf6-7a374a7e3f8f" xlink:href="https://xbrl.sec.gov/currency/2023/currency-2023.xsd#currency_EUR"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_currency_AllCurrenciesDomain_e1101e2f-839e-44e3-97ed-98c3252403d5" xlink:to="loc_currency_EUR_c9f28bd8-90dd-4457-adf6-7a374a7e3f8f" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_currency_CAD_ca311d0f-adb3-437f-aff5-428bf7f941fc" xlink:href="https://xbrl.sec.gov/currency/2023/currency-2023.xsd#currency_CAD"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_currency_AllCurrenciesDomain_e1101e2f-839e-44e3-97ed-98c3252403d5" xlink:to="loc_currency_CAD_ca311d0f-adb3-437f-aff5-428bf7f941fc" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_0f17c5e1-d459-46d9-b7dc-67edfaa845d2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_fc0e3ffc-6a20-43c9-82d0-ae38e92a05e0" xlink:to="loc_us-gaap_DebtInstrumentAxis_0f17c5e1-d459-46d9-b7dc-67edfaa845d2" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_0f17c5e1-d459-46d9-b7dc-67edfaa845d2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_0f17c5e1-d459-46d9-b7dc-67edfaa845d2" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_0f17c5e1-d459-46d9-b7dc-67edfaa845d2_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_2a9a97f6-a6b0-4541-b485-08387a60fba7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_0f17c5e1-d459-46d9-b7dc-67edfaa845d2" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_2a9a97f6-a6b0-4541-b485-08387a60fba7" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_d02964e3-288a-40dd-a60b-9a97ef103165" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevolvingCreditFacilityMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_2a9a97f6-a6b0-4541-b485-08387a60fba7" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_d02964e3-288a-40dd-a60b-9a97ef103165" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_7e9caf9d-5d96-4b60-ae56-b60aa9d20391" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_fc0e3ffc-6a20-43c9-82d0-ae38e92a05e0" xlink:to="loc_srt_RangeAxis_7e9caf9d-5d96-4b60-ae56-b60aa9d20391" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_7e9caf9d-5d96-4b60-ae56-b60aa9d20391_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_7e9caf9d-5d96-4b60-ae56-b60aa9d20391" xlink:to="loc_srt_RangeMember_7e9caf9d-5d96-4b60-ae56-b60aa9d20391_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_b56bc5ef-0e38-41f0-885f-015236dc30e8" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_7e9caf9d-5d96-4b60-ae56-b60aa9d20391" xlink:to="loc_srt_RangeMember_b56bc5ef-0e38-41f0-885f-015236dc30e8" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_acba663e-5a2e-4039-aa01-75178f5bbac5" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MinimumMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_b56bc5ef-0e38-41f0-885f-015236dc30e8" xlink:to="loc_srt_MinimumMember_acba663e-5a2e-4039-aa01-75178f5bbac5" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_ff3de7a0-c8da-43a3-89e9-341b0daf578b" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MaximumMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_b56bc5ef-0e38-41f0-885f-015236dc30e8" xlink:to="loc_srt_MaximumMember_ff3de7a0-c8da-43a3-89e9-341b0daf578b" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_23ca2dee-aef5-475b-ad16-d625009e1710" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongtermDebtTypeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_fc0e3ffc-6a20-43c9-82d0-ae38e92a05e0" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_23ca2dee-aef5-475b-ad16-d625009e1710" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_23ca2dee-aef5-475b-ad16-d625009e1710_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongtermDebtTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_23ca2dee-aef5-475b-ad16-d625009e1710" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_23ca2dee-aef5-475b-ad16-d625009e1710_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_3e981ba2-d5b1-41df-906e-79dc6dd41d99" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongtermDebtTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_23ca2dee-aef5-475b-ad16-d625009e1710" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_3e981ba2-d5b1-41df-906e-79dc6dd41d99" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_f2a295fc-d862-4e2d-913b-2f05db281b98" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LineOfCreditMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_3e981ba2-d5b1-41df-906e-79dc6dd41d99" xlink:to="loc_us-gaap_LineOfCreditMember_f2a295fc-d862-4e2d-913b-2f05db281b98" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_MediumTermNotesMember_800f93eb-fa02-43c8-9b72-df0a51821314" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MediumTermNotesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_3e981ba2-d5b1-41df-906e-79dc6dd41d99" xlink:to="loc_us-gaap_MediumTermNotesMember_800f93eb-fa02-43c8-9b72-df0a51821314" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_7d7d0871-c0d9-4e84-8d5f-1729d9662ffd" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_VariableRateAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_fc0e3ffc-6a20-43c9-82d0-ae38e92a05e0" xlink:to="loc_us-gaap_VariableRateAxis_7d7d0871-c0d9-4e84-8d5f-1729d9662ffd" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_7d7d0871-c0d9-4e84-8d5f-1729d9662ffd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_VariableRateDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_7d7d0871-c0d9-4e84-8d5f-1729d9662ffd" xlink:to="loc_us-gaap_VariableRateDomain_7d7d0871-c0d9-4e84-8d5f-1729d9662ffd_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_7bc7cbfe-2196-4972-9442-e991eb45647e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_VariableRateDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_7d7d0871-c0d9-4e84-8d5f-1729d9662ffd" xlink:to="loc_us-gaap_VariableRateDomain_7bc7cbfe-2196-4972-9442-e991eb45647e" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_BaseRateMember_362ae30c-b756-42c4-9f21-3b053eae522f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BaseRateMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_7bc7cbfe-2196-4972-9442-e991eb45647e" xlink:to="loc_us-gaap_BaseRateMember_362ae30c-b756-42c4-9f21-3b053eae522f" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_SecuredOvernightFinancingRateMember_828b6519-1cf7-4ece-969c-676ceaaffa97" xlink:href="ecvt-20231231.xsd#ecvt_SecuredOvernightFinancingRateMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_7bc7cbfe-2196-4972-9442-e991eb45647e" xlink:to="loc_ecvt_SecuredOvernightFinancingRateMember_828b6519-1cf7-4ece-969c-676ceaaffa97" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_e884d1bf-91b0-4b36-bcca-175f658a155a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5cedb98c-afca-4f9a-a88d-f91a23e135f5" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_e884d1bf-91b0-4b36-bcca-175f658a155a" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityIncreaseDecreaseForPeriodNet_906d8905-de23-462e-8e15-c3f135a78225" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LineOfCreditFacilityIncreaseDecreaseForPeriodNet"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5cedb98c-afca-4f9a-a88d-f91a23e135f5" xlink:to="loc_us-gaap_LineOfCreditFacilityIncreaseDecreaseForPeriodNet_906d8905-de23-462e-8e15-c3f135a78225" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityCommitmentFeePercentage_388f601e-b7ca-4248-b956-716d2e4ad8e3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LineOfCreditFacilityCommitmentFeePercentage"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5cedb98c-afca-4f9a-a88d-f91a23e135f5" xlink:to="loc_us-gaap_LineOfCreditFacilityCommitmentFeePercentage_388f601e-b7ca-4248-b956-716d2e4ad8e3" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_LineofCreditFacilityPotentialCommitmentFeePercentage_566bf37b-3058-45dd-9982-9280f91493d5" xlink:href="ecvt-20231231.xsd#ecvt_LineofCreditFacilityPotentialCommitmentFeePercentage"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5cedb98c-afca-4f9a-a88d-f91a23e135f5" xlink:to="loc_ecvt_LineofCreditFacilityPotentialCommitmentFeePercentage_566bf37b-3058-45dd-9982-9280f91493d5" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_4ee6266f-d607-41a8-830c-359ae6132f37" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5cedb98c-afca-4f9a-a88d-f91a23e135f5" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_4ee6266f-d607-41a8-830c-359ae6132f37" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_DebtInstrumentBasisSpreadOnVariableRateSpreadAdjustment_8a3fc89b-f8ef-4f88-89f0-d8b2900d04aa" xlink:href="ecvt-20231231.xsd#ecvt_DebtInstrumentBasisSpreadOnVariableRateSpreadAdjustment"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5cedb98c-afca-4f9a-a88d-f91a23e135f5" xlink:to="loc_ecvt_DebtInstrumentBasisSpreadOnVariableRateSpreadAdjustment_8a3fc89b-f8ef-4f88-89f0-d8b2900d04aa" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_28bd6c92-a3af-440f-b206-3874a47ab03d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5cedb98c-afca-4f9a-a88d-f91a23e135f5" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_28bd6c92-a3af-440f-b206-3874a47ab03d" xlink:type="arc" order="6"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_79fb6155-5596-4da8-aee7-3ee00fcce0d1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LettersOfCreditOutstandingAmount"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5cedb98c-afca-4f9a-a88d-f91a23e135f5" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_79fb6155-5596-4da8-aee7-3ee00fcce0d1" xlink:type="arc" order="7"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_DebtInstrumentCovenantFixedChargeCoverageRatioMinimum_31c01892-56b0-447c-8c1b-40781d7c0ee3" xlink:href="ecvt-20231231.xsd#ecvt_DebtInstrumentCovenantFixedChargeCoverageRatioMinimum"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5cedb98c-afca-4f9a-a88d-f91a23e135f5" xlink:to="loc_ecvt_DebtInstrumentCovenantFixedChargeCoverageRatioMinimum_31c01892-56b0-447c-8c1b-40781d7c0ee3" xlink:type="arc" order="8"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/LongtermDebt2021TermLoanFacilityDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#LongtermDebt2021TermLoanFacilityDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/LongtermDebt2021TermLoanFacilityDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_aa1731ad-12f1-4e04-9050-d079b12c3ca1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_c8c31128-0e95-4397-a027-1545dae185c6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_aa1731ad-12f1-4e04-9050-d079b12c3ca1" xlink:to="loc_us-gaap_DebtInstrumentTable_c8c31128-0e95-4397-a027-1545dae185c6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_2764e1bc-9f79-4091-a415-6902985a8dc9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_c8c31128-0e95-4397-a027-1545dae185c6" xlink:to="loc_us-gaap_DebtInstrumentAxis_2764e1bc-9f79-4091-a415-6902985a8dc9" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_2764e1bc-9f79-4091-a415-6902985a8dc9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_2764e1bc-9f79-4091-a415-6902985a8dc9" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_2764e1bc-9f79-4091-a415-6902985a8dc9_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_a6d8b790-c910-49f5-b9a6-3aca2e401c10" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_2764e1bc-9f79-4091-a415-6902985a8dc9" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_a6d8b790-c910-49f5-b9a6-3aca2e401c10" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_A2021TermLoanFacilityMember_6c4de495-2868-489c-98ac-38123a6a8367" xlink:href="ecvt-20231231.xsd#ecvt_A2021TermLoanFacilityMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_a6d8b790-c910-49f5-b9a6-3aca2e401c10" xlink:to="loc_ecvt_A2021TermLoanFacilityMember_6c4de495-2868-489c-98ac-38123a6a8367" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_d83a7dce-d3dd-4959-9f44-251dce7b08e9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongtermDebtTypeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_c8c31128-0e95-4397-a027-1545dae185c6" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_d83a7dce-d3dd-4959-9f44-251dce7b08e9" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_d83a7dce-d3dd-4959-9f44-251dce7b08e9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongtermDebtTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_d83a7dce-d3dd-4959-9f44-251dce7b08e9" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_d83a7dce-d3dd-4959-9f44-251dce7b08e9_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_1055ec01-6686-4ca1-99ba-80aadd45a598" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongtermDebtTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_d83a7dce-d3dd-4959-9f44-251dce7b08e9" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_1055ec01-6686-4ca1-99ba-80aadd45a598" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_MediumTermNotesMember_5ab42547-b21a-4af2-9eb0-51733996604d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MediumTermNotesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_1055ec01-6686-4ca1-99ba-80aadd45a598" xlink:to="loc_us-gaap_MediumTermNotesMember_5ab42547-b21a-4af2-9eb0-51733996604d" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_A2021TermLoanFacilityMember_9dc8bc82-88d9-45b5-9d84-71c775ccb2ab" xlink:href="ecvt-20231231.xsd#ecvt_A2021TermLoanFacilityMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_1055ec01-6686-4ca1-99ba-80aadd45a598" xlink:to="loc_ecvt_A2021TermLoanFacilityMember_9dc8bc82-88d9-45b5-9d84-71c775ccb2ab" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_0c287b2b-dffb-4267-9c35-acf07c227ddc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_VariableRateAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_c8c31128-0e95-4397-a027-1545dae185c6" xlink:to="loc_us-gaap_VariableRateAxis_0c287b2b-dffb-4267-9c35-acf07c227ddc" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_0c287b2b-dffb-4267-9c35-acf07c227ddc_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_VariableRateDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_0c287b2b-dffb-4267-9c35-acf07c227ddc" xlink:to="loc_us-gaap_VariableRateDomain_0c287b2b-dffb-4267-9c35-acf07c227ddc_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_816ee680-df7d-48ca-9275-c0e3269b3708" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_VariableRateDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_0c287b2b-dffb-4267-9c35-acf07c227ddc" xlink:to="loc_us-gaap_VariableRateDomain_816ee680-df7d-48ca-9275-c0e3269b3708" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_SecuredOvernightFinancingRateMember_d9604293-c4bd-4a0b-b585-62cdb9d0c0fa" xlink:href="ecvt-20231231.xsd#ecvt_SecuredOvernightFinancingRateMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_816ee680-df7d-48ca-9275-c0e3269b3708" xlink:to="loc_ecvt_SecuredOvernightFinancingRateMember_d9604293-c4bd-4a0b-b585-62cdb9d0c0fa" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_830f512e-80aa-416b-8596-0ee172550ddc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentCarryingAmount"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_aa1731ad-12f1-4e04-9050-d079b12c3ca1" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_830f512e-80aa-416b-8596-0ee172550ddc" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_DebtInstrumentOriginalIssueDiscount_63684b3c-ccf4-46e3-9148-6c722e089fc4" xlink:href="ecvt-20231231.xsd#ecvt_DebtInstrumentOriginalIssueDiscount"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_aa1731ad-12f1-4e04-9050-d079b12c3ca1" xlink:to="loc_ecvt_DebtInstrumentOriginalIssueDiscount_63684b3c-ccf4-46e3-9148-6c722e089fc4" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_DebtInstrumentVariableRateFloor_ed0c066c-7a8c-4006-ac13-f1a58ded50cb" xlink:href="ecvt-20231231.xsd#ecvt_DebtInstrumentVariableRateFloor"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_aa1731ad-12f1-4e04-9050-d079b12c3ca1" xlink:to="loc_ecvt_DebtInstrumentVariableRateFloor_ed0c066c-7a8c-4006-ac13-f1a58ded50cb" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_e8d599ec-bc7b-46b1-8ccb-d287e7690520" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_aa1731ad-12f1-4e04-9050-d079b12c3ca1" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_e8d599ec-bc7b-46b1-8ccb-d287e7690520" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_DebtInstrumentFirstLienLeverageRatio_fb7ff2fc-8d38-41ec-952d-23cc2028b0af" xlink:href="ecvt-20231231.xsd#ecvt_DebtInstrumentFirstLienLeverageRatio"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_aa1731ad-12f1-4e04-9050-d079b12c3ca1" xlink:to="loc_ecvt_DebtInstrumentFirstLienLeverageRatio_fb7ff2fc-8d38-41ec-952d-23cc2028b0af" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_DebtInstrumentScheduledRepaymentPercentageOfPrincipal_5fb5e88c-163d-4d6c-b1f7-b0a2279e5019" xlink:href="ecvt-20231231.xsd#ecvt_DebtInstrumentScheduledRepaymentPercentageOfPrincipal"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_aa1731ad-12f1-4e04-9050-d079b12c3ca1" xlink:to="loc_ecvt_DebtInstrumentScheduledRepaymentPercentageOfPrincipal_5fb5e88c-163d-4d6c-b1f7-b0a2279e5019" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_4749332a-f417-476d-ae21-ec19729e55cb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_aa1731ad-12f1-4e04-9050-d079b12c3ca1" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_4749332a-f417-476d-ae21-ec19729e55cb" xlink:type="arc" order="6"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_MandatoryPaymentsExcessCashFlow_2e4815ae-12f5-481f-87ae-f2bc5b0f21db" xlink:href="ecvt-20231231.xsd#ecvt_MandatoryPaymentsExcessCashFlow"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_aa1731ad-12f1-4e04-9050-d079b12c3ca1" xlink:to="loc_ecvt_MandatoryPaymentsExcessCashFlow_2e4815ae-12f5-481f-87ae-f2bc5b0f21db" xlink:type="arc" order="7"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/LongtermDebtFairValueDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#LongtermDebtFairValueDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/LongtermDebtFairValueDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_ea89cf7f-380b-4aac-8e35-4dd2cdde92bc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_c74c1f99-552b-4391-b41d-5f026b50a5d5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ea89cf7f-380b-4aac-8e35-4dd2cdde92bc" xlink:to="loc_us-gaap_DebtInstrumentTable_c74c1f99-552b-4391-b41d-5f026b50a5d5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_faebe1a2-46a2-4cce-91ea-6b041814e0da" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongtermDebtTypeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_c74c1f99-552b-4391-b41d-5f026b50a5d5" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_faebe1a2-46a2-4cce-91ea-6b041814e0da" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_faebe1a2-46a2-4cce-91ea-6b041814e0da_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongtermDebtTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_faebe1a2-46a2-4cce-91ea-6b041814e0da" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_faebe1a2-46a2-4cce-91ea-6b041814e0da_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_30d385c5-6602-45ca-ad9f-c18901f4e402" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongtermDebtTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_faebe1a2-46a2-4cce-91ea-6b041814e0da" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_30d385c5-6602-45ca-ad9f-c18901f4e402" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_A2021TermLoanFacilityMember_958e8021-e819-49e4-aaed-72b4f3332f14" xlink:href="ecvt-20231231.xsd#ecvt_A2021TermLoanFacilityMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_30d385c5-6602-45ca-ad9f-c18901f4e402" xlink:to="loc_ecvt_A2021TermLoanFacilityMember_958e8021-e819-49e4-aaed-72b4f3332f14" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtFairValue_82df8171-6b41-4870-86d3-10ab45046f1f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongTermDebtFairValue"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ea89cf7f-380b-4aac-8e35-4dd2cdde92bc" xlink:to="loc_us-gaap_LongTermDebtFairValue_82df8171-6b41-4870-86d3-10ab45046f1f" xlink:type="arc" order="0"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/FinancialInstrumentsInterestRateCapAgreementsDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#FinancialInstrumentsInterestRateCapAgreementsDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/FinancialInstrumentsInterestRateCapAgreementsDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLineItems_4297b65c-f21f-4619-afb0-f333a9fd66fb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeTable_f30b2946-4779-4610-92f5-e276ed59fbd5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeLineItems_4297b65c-f21f-4619-afb0-f333a9fd66fb" xlink:to="loc_us-gaap_DerivativeTable_f30b2946-4779-4610-92f5-e276ed59fbd5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_504dbfd1-4b0c-44e1-a1b1-3e42004213c3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_f30b2946-4779-4610-92f5-e276ed59fbd5" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_504dbfd1-4b0c-44e1-a1b1-3e42004213c3" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingRelationshipDomain_504dbfd1-4b0c-44e1-a1b1-3e42004213c3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_HedgingRelationshipDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_504dbfd1-4b0c-44e1-a1b1-3e42004213c3" xlink:to="loc_us-gaap_HedgingRelationshipDomain_504dbfd1-4b0c-44e1-a1b1-3e42004213c3_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingRelationshipDomain_da88fd7c-84d4-4de5-8470-21b8d15a8c4c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_HedgingRelationshipDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_504dbfd1-4b0c-44e1-a1b1-3e42004213c3" xlink:to="loc_us-gaap_HedgingRelationshipDomain_da88fd7c-84d4-4de5-8470-21b8d15a8c4c" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashFlowHedgingMember_6e23e9f0-6a64-429d-b9d1-8183079a8941" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CashFlowHedgingMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingRelationshipDomain_da88fd7c-84d4-4de5-8470-21b8d15a8c4c" xlink:to="loc_us-gaap_CashFlowHedgingMember_6e23e9f0-6a64-429d-b9d1-8183079a8941" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_9f1713e1-99d2-456c-9dcd-c81858503d8c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_f30b2946-4779-4610-92f5-e276ed59fbd5" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_9f1713e1-99d2-456c-9dcd-c81858503d8c" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_9f1713e1-99d2-456c-9dcd-c81858503d8c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeContractTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_9f1713e1-99d2-456c-9dcd-c81858503d8c" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_9f1713e1-99d2-456c-9dcd-c81858503d8c_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_8031a203-77e1-44de-92e7-db374619e459" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeContractTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_9f1713e1-99d2-456c-9dcd-c81858503d8c" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_8031a203-77e1-44de-92e7-db374619e459" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_January2022InterestRateCapExpiringOctober2024Member_0500872d-0b66-45b5-a262-53b0dcb4fffc" xlink:href="ecvt-20231231.xsd#ecvt_January2022InterestRateCapExpiringOctober2024Member"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_8031a203-77e1-44de-92e7-db374619e459" xlink:to="loc_ecvt_January2022InterestRateCapExpiringOctober2024Member_0500872d-0b66-45b5-a262-53b0dcb4fffc" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_January2022InterestRateCapExpiringOctober2025Member_12d18152-bb76-4c5e-a616-38e40ce8e0f9" xlink:href="ecvt-20231231.xsd#ecvt_January2022InterestRateCapExpiringOctober2025Member"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_8031a203-77e1-44de-92e7-db374619e459" xlink:to="loc_ecvt_January2022InterestRateCapExpiringOctober2025Member_12d18152-bb76-4c5e-a616-38e40ce8e0f9" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_November2022InterestRateCapJuly2023ToJuly2024Member_467d0426-05ed-46ea-862f-22e2408c94f3" xlink:href="ecvt-20231231.xsd#ecvt_November2022InterestRateCapJuly2023ToJuly2024Member"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_8031a203-77e1-44de-92e7-db374619e459" xlink:to="loc_ecvt_November2022InterestRateCapJuly2023ToJuly2024Member_467d0426-05ed-46ea-862f-22e2408c94f3" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_November2022InterestRateCapAugust2024ToJuly2026Member_f5be56ed-c0b9-4b4f-8099-9e7b60fc24b9" xlink:href="ecvt-20231231.xsd#ecvt_November2022InterestRateCapAugust2024ToJuly2026Member"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_8031a203-77e1-44de-92e7-db374619e459" xlink:to="loc_ecvt_November2022InterestRateCapAugust2024ToJuly2026Member_f5be56ed-c0b9-4b4f-8099-9e7b60fc24b9" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeNumberOfInstrumentsHeld_b7bfb075-4163-4884-9815-6e64d04e4e5b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeNumberOfInstrumentsHeld"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_4297b65c-f21f-4619-afb0-f333a9fd66fb" xlink:to="loc_us-gaap_DerivativeNumberOfInstrumentsHeld_b7bfb075-4163-4884-9815-6e64d04e4e5b" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_DerivativeNumberOfInstrumentsInEffect_7ee5adf2-295d-446f-bef9-fc8b79df3f1f" xlink:href="ecvt-20231231.xsd#ecvt_DerivativeNumberOfInstrumentsInEffect"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_4297b65c-f21f-4619-afb0-f333a9fd66fb" xlink:to="loc_ecvt_DerivativeNumberOfInstrumentsInEffect_7ee5adf2-295d-446f-bef9-fc8b79df3f1f" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeNotionalAmount_37189176-3ad2-41c5-afda-e1a670c7eaed" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeNotionalAmount"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_4297b65c-f21f-4619-afb0-f333a9fd66fb" xlink:to="loc_us-gaap_DerivativeNotionalAmount_37189176-3ad2-41c5-afda-e1a670c7eaed" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeCostOfHedge_01d058a3-2b29-420e-a951-9386c17695ec" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeCostOfHedge"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_4297b65c-f21f-4619-afb0-f333a9fd66fb" xlink:to="loc_us-gaap_DerivativeCostOfHedge_01d058a3-2b29-420e-a951-9386c17695ec" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeCapInterestRate_c4312a15-f86c-42f0-829e-e33d5a94ef13" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeCapInterestRate"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_4297b65c-f21f-4619-afb0-f333a9fd66fb" xlink:to="loc_us-gaap_DerivativeCapInterestRate_c4312a15-f86c-42f0-829e-e33d5a94ef13" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths_d0329a89-b9b2-4826-bf54-746e37a4686a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_4297b65c-f21f-4619-afb0-f333a9fd66fb" xlink:to="loc_us-gaap_CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths_d0329a89-b9b2-4826-bf54-746e37a4686a" xlink:type="arc" order="5"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/FinancialInstrumentsFairValueDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#FinancialInstrumentsFairValueDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/FinancialInstrumentsFairValueDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativesFairValueLineItems_ec0bb129-4a79-4056-9097-7cac9ce4d672" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativesFairValueLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_529fdfc6-6eb7-4309-948b-95fe23e7043f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_ec0bb129-4a79-4056-9097-7cac9ce4d672" xlink:to="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_529fdfc6-6eb7-4309-948b-95fe23e7043f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_4aa5e78d-e9c5-4061-be44-4dfbe784a1ff" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_HedgingDesignationAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_529fdfc6-6eb7-4309-948b-95fe23e7043f" xlink:to="loc_us-gaap_HedgingDesignationAxis_4aa5e78d-e9c5-4061-be44-4dfbe784a1ff" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_4aa5e78d-e9c5-4061-be44-4dfbe784a1ff_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_HedgingDesignationDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_4aa5e78d-e9c5-4061-be44-4dfbe784a1ff" xlink:to="loc_us-gaap_HedgingDesignationDomain_4aa5e78d-e9c5-4061-be44-4dfbe784a1ff_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_b8d5fe98-bc36-4ad5-b1f2-dc7d279d0c05" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_HedgingDesignationDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_4aa5e78d-e9c5-4061-be44-4dfbe784a1ff" xlink:to="loc_us-gaap_HedgingDesignationDomain_b8d5fe98-bc36-4ad5-b1f2-dc7d279d0c05" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_edbf5b51-2506-452f-b75e-f0ed48fb4da0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DesignatedAsHedgingInstrumentMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_b8d5fe98-bc36-4ad5-b1f2-dc7d279d0c05" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_edbf5b51-2506-452f-b75e-f0ed48fb4da0" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_2976a669-4a53-474c-a1fc-94f36cd699b6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_529fdfc6-6eb7-4309-948b-95fe23e7043f" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_2976a669-4a53-474c-a1fc-94f36cd699b6" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingRelationshipDomain_2976a669-4a53-474c-a1fc-94f36cd699b6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_HedgingRelationshipDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_2976a669-4a53-474c-a1fc-94f36cd699b6" xlink:to="loc_us-gaap_HedgingRelationshipDomain_2976a669-4a53-474c-a1fc-94f36cd699b6_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingRelationshipDomain_e8669bdc-cd96-4d97-abb3-2ef626f9604d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_HedgingRelationshipDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_2976a669-4a53-474c-a1fc-94f36cd699b6" xlink:to="loc_us-gaap_HedgingRelationshipDomain_e8669bdc-cd96-4d97-abb3-2ef626f9604d" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashFlowHedgingMember_c54f33c7-ef02-4ea3-a70a-0c5cf99da736" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CashFlowHedgingMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingRelationshipDomain_e8669bdc-cd96-4d97-abb3-2ef626f9604d" xlink:to="loc_us-gaap_CashFlowHedgingMember_c54f33c7-ef02-4ea3-a70a-0c5cf99da736" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_3033f083-a167-48a8-9fda-a3d37e450800" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_529fdfc6-6eb7-4309-948b-95fe23e7043f" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_3033f083-a167-48a8-9fda-a3d37e450800" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_3033f083-a167-48a8-9fda-a3d37e450800_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeContractTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_3033f083-a167-48a8-9fda-a3d37e450800" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_3033f083-a167-48a8-9fda-a3d37e450800_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_5b0b0252-3331-40d8-ad6b-d2558c599f4b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeContractTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_3033f083-a167-48a8-9fda-a3d37e450800" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_5b0b0252-3331-40d8-ad6b-d2558c599f4b" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateCapMember_8ef5267b-48e4-4e76-8edd-40e467a3d007" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_InterestRateCapMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_5b0b0252-3331-40d8-ad6b-d2558c599f4b" xlink:to="loc_us-gaap_InterestRateCapMember_8ef5267b-48e4-4e76-8edd-40e467a3d007" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssetsAbstract_0983c811-c75a-41d6-8fd6-1866f551aab4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeAssetsAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_ec0bb129-4a79-4056-9097-7cac9ce4d672" xlink:to="loc_us-gaap_DerivativeAssetsAbstract_0983c811-c75a-41d6-8fd6-1866f551aab4" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssetCurrentStatementOfFinancialPositionExtensibleEnumeration_bab2e214-407d-4f67-ae6d-86278178583a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeAssetCurrentStatementOfFinancialPositionExtensibleEnumeration"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeAssetsAbstract_0983c811-c75a-41d6-8fd6-1866f551aab4" xlink:to="loc_us-gaap_DerivativeAssetCurrentStatementOfFinancialPositionExtensibleEnumeration_bab2e214-407d-4f67-ae6d-86278178583a" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssetNoncurrentStatementOfFinancialPositionExtensibleEnumeration_4be22478-42f7-4e69-9442-093f12ec4b5d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeAssetNoncurrentStatementOfFinancialPositionExtensibleEnumeration"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeAssetsAbstract_0983c811-c75a-41d6-8fd6-1866f551aab4" xlink:to="loc_us-gaap_DerivativeAssetNoncurrentStatementOfFinancialPositionExtensibleEnumeration_4be22478-42f7-4e69-9442-093f12ec4b5d" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssetsCurrent_acd5190a-7243-424c-b5f6-70b5aeb458d9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeAssetsCurrent"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeAssetsAbstract_0983c811-c75a-41d6-8fd6-1866f551aab4" xlink:to="loc_us-gaap_DerivativeAssetsCurrent_acd5190a-7243-424c-b5f6-70b5aeb458d9" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssetsNoncurrent_9612e31b-4c52-4106-b637-d352c9f1e030" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeAssetsNoncurrent"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeAssetsAbstract_0983c811-c75a-41d6-8fd6-1866f551aab4" xlink:to="loc_us-gaap_DerivativeAssetsNoncurrent_9612e31b-4c52-4106-b637-d352c9f1e030" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssets_74c35840-dead-40e8-86a7-78227395d868" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeAssets"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeAssetsAbstract_0983c811-c75a-41d6-8fd6-1866f551aab4" xlink:to="loc_us-gaap_DerivativeAssets_74c35840-dead-40e8-86a7-78227395d868" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilityFairValueNetAbstract_ad33e681-ecd7-4463-a7b9-1effff838e73" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeLiabilityFairValueNetAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_ec0bb129-4a79-4056-9097-7cac9ce4d672" xlink:to="loc_us-gaap_DerivativeLiabilityFairValueNetAbstract_ad33e681-ecd7-4463-a7b9-1effff838e73" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilityNoncurrentStatementOfFinancialPositionExtensibleEnumeration_b155dbce-79be-4ccd-a714-069d473c6c0e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeLiabilityNoncurrentStatementOfFinancialPositionExtensibleEnumeration"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLiabilityFairValueNetAbstract_ad33e681-ecd7-4463-a7b9-1effff838e73" xlink:to="loc_us-gaap_DerivativeLiabilityNoncurrentStatementOfFinancialPositionExtensibleEnumeration_b155dbce-79be-4ccd-a714-069d473c6c0e" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilitiesNoncurrent_bc6a6c11-aed0-4c61-bbca-3ac6c783b7be" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeLiabilitiesNoncurrent"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLiabilityFairValueNetAbstract_ad33e681-ecd7-4463-a7b9-1effff838e73" xlink:to="loc_us-gaap_DerivativeLiabilitiesNoncurrent_bc6a6c11-aed0-4c61-bbca-3ac6c783b7be" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilities_a8edc109-88d3-447e-be53-a2f9f6e5ec69" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeLiabilities"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLiabilityFairValueNetAbstract_ad33e681-ecd7-4463-a7b9-1effff838e73" xlink:to="loc_us-gaap_DerivativeLiabilities_a8edc109-88d3-447e-be53-a2f9f6e5ec69" xlink:type="arc" order="3"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/FinancialInstrumentsEffectonOtherComprehensiveIncomeDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#FinancialInstrumentsEffectonOtherComprehensiveIncomeDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/FinancialInstrumentsEffectonOtherComprehensiveIncomeDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossLineItems_e56e9437-16fc-428a-b845-86544d3c4dd0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeInstrumentsGainLossLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable_e1ec3a7c-ab47-47df-bb0f-41eb00b4423f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossLineItems_e56e9437-16fc-428a-b845-86544d3c4dd0" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable_e1ec3a7c-ab47-47df-bb0f-41eb00b4423f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_a96a96fa-5e74-4370-b14b-be1a0b08a9c2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable_e1ec3a7c-ab47-47df-bb0f-41eb00b4423f" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_a96a96fa-5e74-4370-b14b-be1a0b08a9c2" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_a96a96fa-5e74-4370-b14b-be1a0b08a9c2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeContractTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_a96a96fa-5e74-4370-b14b-be1a0b08a9c2" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_a96a96fa-5e74-4370-b14b-be1a0b08a9c2_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_bf20aa00-5bc5-4db1-9463-bb5887a6879a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeContractTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_a96a96fa-5e74-4370-b14b-be1a0b08a9c2" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_bf20aa00-5bc5-4db1-9463-bb5887a6879a" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateCapMember_28cd2b35-fa22-481a-ba4a-e37763c69761" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_InterestRateCapMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_bf20aa00-5bc5-4db1-9463-bb5887a6879a" xlink:to="loc_us-gaap_InterestRateCapMember_28cd2b35-fa22-481a-ba4a-e37763c69761" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_a159f96a-c73e-4d53-bb9f-54662655663f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeStatementLocationAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable_e1ec3a7c-ab47-47df-bb0f-41eb00b4423f" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_a159f96a-c73e-4d53-bb9f-54662655663f" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_a159f96a-c73e-4d53-bb9f-54662655663f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeStatementLocationDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_a159f96a-c73e-4d53-bb9f-54662655663f" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_a159f96a-c73e-4d53-bb9f-54662655663f_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_fbb7c617-1244-4db8-aa5e-73da70d1eee4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeStatementLocationDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_a159f96a-c73e-4d53-bb9f-54662655663f" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_fbb7c617-1244-4db8-aa5e-73da70d1eee4" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseMember_b62af47a-b573-4853-bf68-a71ae598099c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_InterestExpenseMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_fbb7c617-1244-4db8-aa5e-73da70d1eee4" xlink:to="loc_us-gaap_InterestExpenseMember_b62af47a-b573-4853-bf68-a71ae598099c" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeDerivativesQualifyingAsHedgesNetOfTaxPeriodIncreaseDecreaseAbstract_0b26554a-0dfc-46b7-851b-caa3a6fc7764" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeDerivativesQualifyingAsHedgesNetOfTaxPeriodIncreaseDecreaseAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossLineItems_e56e9437-16fc-428a-b845-86544d3c4dd0" xlink:to="loc_us-gaap_OtherComprehensiveIncomeDerivativesQualifyingAsHedgesNetOfTaxPeriodIncreaseDecreaseAbstract_0b26554a-0dfc-46b7-851b-caa3a6fc7764" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAfterTax_36b44d8b-252b-45ab-ac38-a550ab70884a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAfterTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeDerivativesQualifyingAsHedgesNetOfTaxPeriodIncreaseDecreaseAbstract_0b26554a-0dfc-46b7-851b-caa3a6fc7764" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAfterTax_36b44d8b-252b-45ab-ac38-a550ab70884a" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNetAbstract_e83fefff-72bd-416a-9d5b-d6c8c9fbbccc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNetAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossLineItems_e56e9437-16fc-428a-b845-86544d3c4dd0" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNetAbstract_e83fefff-72bd-416a-9d5b-d6c8c9fbbccc" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationAfterTax_e98d7375-f91b-49bd-9b18-52aba3f2d774" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationAfterTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNetAbstract_e83fefff-72bd-416a-9d5b-d6c8c9fbbccc" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationAfterTax_e98d7375-f91b-49bd-9b18-52aba3f2d774" xlink:type="arc" order="1"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/FinancialInstrumentsCashFlowHedgeImpactonStatementsofIncomeDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#FinancialInstrumentsCashFlowHedgeImpactonStatementsofIncomeDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/FinancialInstrumentsCashFlowHedgeImpactonStatementsofIncomeDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLineItems_eec37a9d-8f4e-4082-8f67-693ba8973989" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeTable_a820e16f-f27b-4e04-9f03-141459864f1b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeLineItems_eec37a9d-8f4e-4082-8f67-693ba8973989" xlink:to="loc_us-gaap_DerivativeTable_a820e16f-f27b-4e04-9f03-141459864f1b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_c1834efb-4bec-43b9-b6a3-64184722f95f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementEquityComponentsAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_a820e16f-f27b-4e04-9f03-141459864f1b" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_c1834efb-4bec-43b9-b6a3-64184722f95f" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_c1834efb-4bec-43b9-b6a3-64184722f95f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_c1834efb-4bec-43b9-b6a3-64184722f95f" xlink:to="loc_us-gaap_EquityComponentDomain_c1834efb-4bec-43b9-b6a3-64184722f95f_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_2b54cfef-c744-4cae-8efd-6b6d8814fe64" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_c1834efb-4bec-43b9-b6a3-64184722f95f" xlink:to="loc_us-gaap_EquityComponentDomain_2b54cfef-c744-4cae-8efd-6b6d8814fe64" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedGainLossCashFlowHedgeIncludingNoncontrollingInterestMember_deecb3f9-eb16-4e12-a534-e0630b63b603" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedGainLossCashFlowHedgeIncludingNoncontrollingInterestMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_2b54cfef-c744-4cae-8efd-6b6d8814fe64" xlink:to="loc_us-gaap_AccumulatedGainLossCashFlowHedgeIncludingNoncontrollingInterestMember_deecb3f9-eb16-4e12-a534-e0630b63b603" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_9cbb52fa-13ac-44d2-b503-50a6998bd35b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_a820e16f-f27b-4e04-9f03-141459864f1b" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_9cbb52fa-13ac-44d2-b503-50a6998bd35b" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_9cbb52fa-13ac-44d2-b503-50a6998bd35b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_9cbb52fa-13ac-44d2-b503-50a6998bd35b" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_9cbb52fa-13ac-44d2-b503-50a6998bd35b_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_9bddeb36-c599-4115-9900-ca45f7a3ed93" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_9cbb52fa-13ac-44d2-b503-50a6998bd35b" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_9bddeb36-c599-4115-9900-ca45f7a3ed93" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_533c3024-f6d1-4c27-a9ba-01efb33d73d6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_9bddeb36-c599-4115-9900-ca45f7a3ed93" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_533c3024-f6d1-4c27-a9ba-01efb33d73d6" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_93cfa2f6-44b2-4ae5-a457-a11c3eb42a9c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_InterestExpense"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_eec37a9d-8f4e-4082-8f67-693ba8973989" xlink:to="loc_us-gaap_InterestExpense_93cfa2f6-44b2-4ae5-a457-a11c3eb42a9c" xlink:type="arc" order="0"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/IncomeTaxesNarrativesDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#IncomeTaxesNarrativesDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/IncomeTaxesNarrativesDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwardsLineItems_6a0a4e85-be16-4249-8671-8616505bfc19" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OperatingLossCarryforwardsLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwardsTable_3b2a8e0b-a321-45da-ad82-66f51ff933de" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OperatingLossCarryforwardsTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_6a0a4e85-be16-4249-8671-8616505bfc19" xlink:to="loc_us-gaap_OperatingLossCarryforwardsTable_3b2a8e0b-a321-45da-ad82-66f51ff933de" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesTypeAxis_82de6b29-3860-40e7-8ba9-a174ba390a99" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ValuationAllowancesAndReservesTypeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OperatingLossCarryforwardsTable_3b2a8e0b-a321-45da-ad82-66f51ff933de" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesTypeAxis_82de6b29-3860-40e7-8ba9-a174ba390a99" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesDomain_82de6b29-3860-40e7-8ba9-a174ba390a99_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ValuationAllowancesAndReservesDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ValuationAllowancesAndReservesTypeAxis_82de6b29-3860-40e7-8ba9-a174ba390a99" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesDomain_82de6b29-3860-40e7-8ba9-a174ba390a99_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesDomain_397879d7-4264-41c9-a4eb-8aee14e8c738" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ValuationAllowancesAndReservesDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ValuationAllowancesAndReservesTypeAxis_82de6b29-3860-40e7-8ba9-a174ba390a99" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesDomain_397879d7-4264-41c9-a4eb-8aee14e8c738" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowanceOfDeferredTaxAssetsMember_96ce9b8e-7788-4364-b3df-8c63eb0dd667" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ValuationAllowanceOfDeferredTaxAssetsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationAllowancesAndReservesDomain_397879d7-4264-41c9-a4eb-8aee14e8c738" xlink:to="loc_us-gaap_ValuationAllowanceOfDeferredTaxAssetsMember_96ce9b8e-7788-4364-b3df-8c63eb0dd667" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_9b00f71c-081a-4176-8439-da6d113ff4eb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BalanceSheetLocationAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OperatingLossCarryforwardsTable_3b2a8e0b-a321-45da-ad82-66f51ff933de" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_9b00f71c-081a-4176-8439-da6d113ff4eb" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_9b00f71c-081a-4176-8439-da6d113ff4eb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BalanceSheetLocationDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_9b00f71c-081a-4176-8439-da6d113ff4eb" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_9b00f71c-081a-4176-8439-da6d113ff4eb_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_cac24132-2398-43a5-8f8f-55abcfaa9c61" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BalanceSheetLocationDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_9b00f71c-081a-4176-8439-da6d113ff4eb" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_cac24132-2398-43a5-8f8f-55abcfaa9c61" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNoncurrentLiabilitiesMember_a1e37f62-f477-4ee8-a0c0-ab9a1172aebd" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherNoncurrentLiabilitiesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_cac24132-2398-43a5-8f8f-55abcfaa9c61" xlink:to="loc_us-gaap_OtherNoncurrentLiabilitiesMember_a1e37f62-f477-4ee8-a0c0-ab9a1172aebd" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_DeferredTaxAssetsTaxCreditWithholdingsForeign_769f188d-b132-46b8-9c97-679482f0796d" xlink:href="ecvt-20231231.xsd#ecvt_DeferredTaxAssetsTaxCreditWithholdingsForeign"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_6a0a4e85-be16-4249-8671-8616505bfc19" xlink:to="loc_ecvt_DeferredTaxAssetsTaxCreditWithholdingsForeign_769f188d-b132-46b8-9c97-679482f0796d" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsDerivativeInstruments_6d329954-36a6-40f1-a459-725c5862b008" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DeferredTaxAssetsDerivativeInstruments"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_6a0a4e85-be16-4249-8671-8616505bfc19" xlink:to="loc_us-gaap_DeferredTaxAssetsDerivativeInstruments_6d329954-36a6-40f1-a459-725c5862b008" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxCreditCarryforwardValuationAllowance_1343e726-def9-48cd-92cf-2629baa86095" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TaxCreditCarryforwardValuationAllowance"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_6a0a4e85-be16-4249-8671-8616505bfc19" xlink:to="loc_us-gaap_TaxCreditCarryforwardValuationAllowance_1343e726-def9-48cd-92cf-2629baa86095" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwards_5456e1b9-dc70-4dac-aed5-939e71593070" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DeferredTaxAssetsOperatingLossCarryforwards"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_6a0a4e85-be16-4249-8671-8616505bfc19" xlink:to="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwards_5456e1b9-dc70-4dac-aed5-939e71593070" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwardsValuationAllowance_0eca231d-f7ab-4b45-afd6-2bdbb73b6209" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OperatingLossCarryforwardsValuationAllowance"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_6a0a4e85-be16-4249-8671-8616505bfc19" xlink:to="loc_us-gaap_OperatingLossCarryforwardsValuationAllowance_0eca231d-f7ab-4b45-afd6-2bdbb73b6209" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesPeriodIncreaseDecrease_ff5e8fa1-c2bf-400f-a2e4-c6059dfdade2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ValuationAllowancesAndReservesPeriodIncreaseDecrease"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_6a0a4e85-be16-4249-8671-8616505bfc19" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesPeriodIncreaseDecrease_ff5e8fa1-c2bf-400f-a2e4-c6059dfdade2" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_UndistributedEarningsOfForeignSubsidiaries_4fb710ff-1ebf-40ed-a0d3-c4a069e20650" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_UndistributedEarningsOfForeignSubsidiaries"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_6a0a4e85-be16-4249-8671-8616505bfc19" xlink:to="loc_us-gaap_UndistributedEarningsOfForeignSubsidiaries_4fb710ff-1ebf-40ed-a0d3-c4a069e20650" xlink:type="arc" order="6"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense_1d80c627-b8e6-449c-abc2-427a955bab15" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_6a0a4e85-be16-4249-8671-8616505bfc19" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense_1d80c627-b8e6-449c-abc2-427a955bab15" xlink:type="arc" order="7"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued_a6ca70b0-966e-41b6-9b03-f8170e08439b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_6a0a4e85-be16-4249-8671-8616505bfc19" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued_a6ca70b0-966e-41b6-9b03-f8170e08439b" xlink:type="arc" order="8"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate_0b6ec39e-0f1e-4444-9d75-a27bf0173cfa" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_6a0a4e85-be16-4249-8671-8616505bfc19" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate_0b6ec39e-0f1e-4444-9d75-a27bf0173cfa" xlink:type="arc" order="9"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_InflationReductionActCorporateAlternativeMinimumTax_6210b8dd-706d-479e-a221-9f317e5224ba" xlink:href="ecvt-20231231.xsd#ecvt_InflationReductionActCorporateAlternativeMinimumTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_6a0a4e85-be16-4249-8671-8616505bfc19" xlink:to="loc_ecvt_InflationReductionActCorporateAlternativeMinimumTax_6210b8dd-706d-479e-a221-9f317e5224ba" xlink:type="arc" order="10"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_InflationReductionActAverageAnnualAdjustedIncomeExcess_e3316387-1d54-452c-ab9e-32a7e8ad06e2" xlink:href="ecvt-20231231.xsd#ecvt_InflationReductionActAverageAnnualAdjustedIncomeExcess"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_6a0a4e85-be16-4249-8671-8616505bfc19" xlink:to="loc_ecvt_InflationReductionActAverageAnnualAdjustedIncomeExcess_e3316387-1d54-452c-ab9e-32a7e8ad06e2" xlink:type="arc" order="11"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_InflationReductionActExciseTaxOnNetStockRepurchases_61c17f61-069f-4b6b-8e59-8dd98464e467" xlink:href="ecvt-20231231.xsd#ecvt_InflationReductionActExciseTaxOnNetStockRepurchases"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_6a0a4e85-be16-4249-8671-8616505bfc19" xlink:to="loc_ecvt_InflationReductionActExciseTaxOnNetStockRepurchases_61c17f61-069f-4b6b-8e59-8dd98464e467" xlink:type="arc" order="12"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/IncomeTaxesPaymentforTaxesDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#IncomeTaxesPaymentforTaxesDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/IncomeTaxesPaymentforTaxesDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwardsLineItems_c980d17d-eda4-44bf-bcf3-49c832e01a59" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OperatingLossCarryforwardsLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwardsTable_3ca70c69-51b6-45af-aa1d-2fa0490aa0d5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OperatingLossCarryforwardsTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_c980d17d-eda4-44bf-bcf3-49c832e01a59" xlink:to="loc_us-gaap_OperatingLossCarryforwardsTable_3ca70c69-51b6-45af-aa1d-2fa0490aa0d5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_1caf0688-706e-44b2-9bc7-e808f09400f5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeTaxAuthorityAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OperatingLossCarryforwardsTable_3ca70c69-51b6-45af-aa1d-2fa0490aa0d5" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_1caf0688-706e-44b2-9bc7-e808f09400f5" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_1caf0688-706e-44b2-9bc7-e808f09400f5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeTaxAuthorityDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_1caf0688-706e-44b2-9bc7-e808f09400f5" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_1caf0688-706e-44b2-9bc7-e808f09400f5_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_87de0fc9-2e92-4d0d-8a2a-cf219793fcac" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeTaxAuthorityDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_1caf0688-706e-44b2-9bc7-e808f09400f5" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_87de0fc9-2e92-4d0d-8a2a-cf219793fcac" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticCountryMember_1155ba23-e1ad-4591-a9cd-59831a43a05c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DomesticCountryMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_87de0fc9-2e92-4d0d-8a2a-cf219793fcac" xlink:to="loc_us-gaap_DomesticCountryMember_1155ba23-e1ad-4591-a9cd-59831a43a05c" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCountryMember_b6dfadab-a224-415a-876a-89afe1c5159d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ForeignCountryMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_87de0fc9-2e92-4d0d-8a2a-cf219793fcac" xlink:to="loc_us-gaap_ForeignCountryMember_b6dfadab-a224-415a-876a-89afe1c5159d" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_1d832ae9-ffb9-42aa-a95f-e380b19edbec" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementOperatingActivitiesSegmentAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OperatingLossCarryforwardsTable_3ca70c69-51b6-45af-aa1d-2fa0490aa0d5" xlink:to="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_1d832ae9-ffb9-42aa-a95f-e380b19edbec" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentOperatingActivitiesDomain_1d832ae9-ffb9-42aa-a95f-e380b19edbec_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentOperatingActivitiesDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_1d832ae9-ffb9-42aa-a95f-e380b19edbec" xlink:to="loc_us-gaap_SegmentOperatingActivitiesDomain_1d832ae9-ffb9-42aa-a95f-e380b19edbec_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentOperatingActivitiesDomain_b34dcc69-3549-4141-940d-fc537d569721" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentOperatingActivitiesDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_1d832ae9-ffb9-42aa-a95f-e380b19edbec" xlink:to="loc_us-gaap_SegmentOperatingActivitiesDomain_b34dcc69-3549-4141-940d-fc537d569721" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentContinuingOperationsMember_2b2f400b-0dcf-44a0-be51-ce0cd97dd761" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentContinuingOperationsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentOperatingActivitiesDomain_b34dcc69-3549-4141-940d-fc537d569721" xlink:to="loc_us-gaap_SegmentContinuingOperationsMember_2b2f400b-0dcf-44a0-be51-ce0cd97dd761" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxesPaidNet_bc961ee4-fcd6-4fb2-af49-e412c5e9b8a6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeTaxesPaidNet"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_c980d17d-eda4-44bf-bcf3-49c832e01a59" xlink:to="loc_us-gaap_IncomeTaxesPaidNet_bc961ee4-fcd6-4fb2-af49-e412c5e9b8a6" xlink:type="arc" order="0"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/BenefitPlansChangeinBenefitObligationandPlanAssetsDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#BenefitPlansChangeinBenefitObligationandPlanAssetsDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/BenefitPlansChangeinBenefitObligationandPlanAssetsDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_19936c96-26c7-4e9d-b7e4-dcb70c994779" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_79fbc8ac-7085-4367-9562-8862e0646dc1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_19936c96-26c7-4e9d-b7e4-dcb70c994779" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_79fbc8ac-7085-4367-9562-8862e0646dc1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_dd53d0d7-2815-4202-a07a-0347a166259e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_79fbc8ac-7085-4367-9562-8862e0646dc1" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_dd53d0d7-2815-4202-a07a-0347a166259e" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_dd53d0d7-2815-4202-a07a-0347a166259e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_dd53d0d7-2815-4202-a07a-0347a166259e" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_dd53d0d7-2815-4202-a07a-0347a166259e_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_647987ac-b007-47f6-9e08-471ea2a457f6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_dd53d0d7-2815-4202-a07a-0347a166259e" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_647987ac-b007-47f6-9e08-471ea2a457f6" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PensionPlansDefinedBenefitMember_316e6813-16fc-4dc2-842c-3d66391f42ba" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PensionPlansDefinedBenefitMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_647987ac-b007-47f6-9e08-471ea2a457f6" xlink:to="loc_us-gaap_PensionPlansDefinedBenefitMember_316e6813-16fc-4dc2-842c-3d66391f42ba" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnfundedPlanMember_593beae8-d832-423a-8627-2bb2546fcaa0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_UnfundedPlanMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_647987ac-b007-47f6-9e08-471ea2a457f6" xlink:to="loc_us-gaap_UnfundedPlanMember_593beae8-d832-423a-8627-2bb2546fcaa0" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_9963c082-edc2-4add-8511-01057c75d9c5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_647987ac-b007-47f6-9e08-471ea2a457f6" xlink:to="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_9963c082-edc2-4add-8511-01057c75d9c5" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_deaec5f6-f99c-4ee6-b8a2-1c0b474978c2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_19936c96-26c7-4e9d-b7e4-dcb70c994779" xlink:to="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_deaec5f6-f99c-4ee6-b8a2-1c0b474978c2" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanBenefitObligation_cf472a64-2cfe-4591-98f1-0ed11ec776b1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanBenefitObligation"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_deaec5f6-f99c-4ee6-b8a2-1c0b474978c2" xlink:to="loc_us-gaap_DefinedBenefitPlanBenefitObligation_cf472a64-2cfe-4591-98f1-0ed11ec776b1" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanInterestCost_cf7a2a21-1712-47fc-973f-69c3c61d7caf" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanInterestCost"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_deaec5f6-f99c-4ee6-b8a2-1c0b474978c2" xlink:to="loc_us-gaap_DefinedBenefitPlanInterestCost_cf7a2a21-1712-47fc-973f-69c3c61d7caf" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanSettlementsBenefitObligation_f77d50a3-2856-4b61-935a-8860639304c5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanSettlementsBenefitObligation"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_deaec5f6-f99c-4ee6-b8a2-1c0b474978c2" xlink:to="loc_us-gaap_DefinedBenefitPlanSettlementsBenefitObligation_f77d50a3-2856-4b61-935a-8860639304c5" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanBenefitObligationBenefitsPaid_70ff4ec7-7a13-466a-88b3-fa5b02d27ccf" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanBenefitObligationBenefitsPaid"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_deaec5f6-f99c-4ee6-b8a2-1c0b474978c2" xlink:to="loc_us-gaap_DefinedBenefitPlanBenefitObligationBenefitsPaid_70ff4ec7-7a13-466a-88b3-fa5b02d27ccf" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_DefinedBenefitPlanBenefitObligationPremiumsPaid_4e4b3455-a643-4e8c-bcdb-378942303a45" xlink:href="ecvt-20231231.xsd#ecvt_DefinedBenefitPlanBenefitObligationPremiumsPaid"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_deaec5f6-f99c-4ee6-b8a2-1c0b474978c2" xlink:to="loc_ecvt_DefinedBenefitPlanBenefitObligationPremiumsPaid_4e4b3455-a643-4e8c-bcdb-378942303a45" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanActuarialGainLoss_44009726-f763-47ea-95ed-822a41f3b213" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanActuarialGainLoss"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward_deaec5f6-f99c-4ee6-b8a2-1c0b474978c2" xlink:to="loc_us-gaap_DefinedBenefitPlanActuarialGainLoss_44009726-f763-47ea-95ed-822a41f3b213" xlink:type="arc" order="6"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanBenefitObligation_492c2b4f-679b-4c25-8aab-d6af685ffbd3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanBenefitObligation"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_ActuarialGainLossChangesduetoDiscountRates_1aacbdbb-4885-4baf-bcb7-e8ae6f20572c" xlink:href="ecvt-20231231.xsd#ecvt_ActuarialGainLossChangesduetoDiscountRates"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_19936c96-26c7-4e9d-b7e4-dcb70c994779" xlink:to="loc_ecvt_ActuarialGainLossChangesduetoDiscountRates_1aacbdbb-4885-4baf-bcb7-e8ae6f20572c" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_ActuarialGainLossGeneralExperience_7e46b60a-8ae3-41cb-97ac-61f1a13d9547" xlink:href="ecvt-20231231.xsd#ecvt_ActuarialGainLossGeneralExperience"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_19936c96-26c7-4e9d-b7e4-dcb70c994779" xlink:to="loc_ecvt_ActuarialGainLossGeneralExperience_7e46b60a-8ae3-41cb-97ac-61f1a13d9547" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_4952585c-4917-41f9-9633-ce24a8a4fca6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_19936c96-26c7-4e9d-b7e4-dcb70c994779" xlink:to="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_4952585c-4917-41f9-9633-ce24a8a4fca6" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanContributionsByEmployer_4b23f08e-5029-41eb-a2c4-a4623ba27496" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanContributionsByEmployer"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_4952585c-4917-41f9-9633-ce24a8a4fca6" xlink:to="loc_us-gaap_DefinedBenefitPlanContributionsByEmployer_4b23f08e-5029-41eb-a2c4-a4623ba27496" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_50a26a77-a468-445c-a23b-c44e9b1b4b63" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanFairValueOfPlanAssets"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_4952585c-4917-41f9-9633-ce24a8a4fca6" xlink:to="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_50a26a77-a468-445c-a23b-c44e9b1b4b63" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets_a97510a2-a140-44e1-baf4-a01086fc55cd" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_4952585c-4917-41f9-9633-ce24a8a4fca6" xlink:to="loc_us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets_a97510a2-a140-44e1-baf4-a01086fc55cd" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanSettlementsPlanAssets_96a553f8-8436-407a-b520-82ba6900eca0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanSettlementsPlanAssets"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_4952585c-4917-41f9-9633-ce24a8a4fca6" xlink:to="loc_us-gaap_DefinedBenefitPlanSettlementsPlanAssets_96a553f8-8436-407a-b520-82ba6900eca0" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanPlanAssetsBenefitsPaid_c2170135-8731-4084-b5ec-640db787f629" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanPlanAssetsBenefitsPaid"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_4952585c-4917-41f9-9633-ce24a8a4fca6" xlink:to="loc_us-gaap_DefinedBenefitPlanPlanAssetsBenefitsPaid_c2170135-8731-4084-b5ec-640db787f629" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_DefinedBenefitPlanPlanAssetsPremiumsPaid_b7f92a2c-205c-4d6f-bc26-f48b0a6b6f12" xlink:href="ecvt-20231231.xsd#ecvt_DefinedBenefitPlanPlanAssetsPremiumsPaid"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward_4952585c-4917-41f9-9633-ce24a8a4fca6" xlink:to="loc_ecvt_DefinedBenefitPlanPlanAssetsPremiumsPaid_b7f92a2c-205c-4d6f-bc26-f48b0a6b6f12" xlink:type="arc" order="6"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_0265ba61-5331-4cf5-93b0-52b1c63972f6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanFairValueOfPlanAssets"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanFundedStatusOfPlan_2dd7cb5e-7303-41b2-8eba-07409a7185a7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanFundedStatusOfPlan"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_19936c96-26c7-4e9d-b7e4-dcb70c994779" xlink:to="loc_us-gaap_DefinedBenefitPlanFundedStatusOfPlan_2dd7cb5e-7303-41b2-8eba-07409a7185a7" xlink:type="arc" order="4"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/BenefitPlansRecognizedontheBalanceSheetDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#BenefitPlansRecognizedontheBalanceSheetDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/BenefitPlansRecognizedontheBalanceSheetDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_a30f5e34-76d8-4a2f-9aa2-26bfa14701a5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_2121583f-a14a-4c19-b19d-f9a5ab335a10" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_a30f5e34-76d8-4a2f-9aa2-26bfa14701a5" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_2121583f-a14a-4c19-b19d-f9a5ab335a10" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_9750eba5-62fb-4e56-b395-bc7d39b7f494" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_2121583f-a14a-4c19-b19d-f9a5ab335a10" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_9750eba5-62fb-4e56-b395-bc7d39b7f494" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_9750eba5-62fb-4e56-b395-bc7d39b7f494_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_9750eba5-62fb-4e56-b395-bc7d39b7f494" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_9750eba5-62fb-4e56-b395-bc7d39b7f494_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_6a03f84f-248f-4bab-a713-118427a919b0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_9750eba5-62fb-4e56-b395-bc7d39b7f494" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_6a03f84f-248f-4bab-a713-118427a919b0" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PensionPlansDefinedBenefitMember_a5e1f6bf-a24e-47df-8bc1-b7514dd3a196" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PensionPlansDefinedBenefitMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_6a03f84f-248f-4bab-a713-118427a919b0" xlink:to="loc_us-gaap_PensionPlansDefinedBenefitMember_a5e1f6bf-a24e-47df-8bc1-b7514dd3a196" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_e35f3a4b-f248-41ac-8852-eb16b5a76826" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_6a03f84f-248f-4bab-a713-118427a919b0" xlink:to="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_e35f3a4b-f248-41ac-8852-eb16b5a76826" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheetAbstract_8cf982ab-fe08-47e7-ae1b-f8b4fb19f05d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheetAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_a30f5e34-76d8-4a2f-9aa2-26bfa14701a5" xlink:to="loc_us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheetAbstract_8cf982ab-fe08-47e7-ae1b-f8b4fb19f05d" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansCurrentLiabilities_626bc039-77c7-489b-a629-fe02f434b803" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansCurrentLiabilities"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheetAbstract_8cf982ab-fe08-47e7-ae1b-f8b4fb19f05d" xlink:to="loc_us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansCurrentLiabilities_626bc039-77c7-489b-a629-fe02f434b803" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent_f3fe9ab0-0657-47ee-a206-d46525ab269d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheetAbstract_8cf982ab-fe08-47e7-ae1b-f8b4fb19f05d" xlink:to="loc_us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent_f3fe9ab0-0657-47ee-a206-d46525ab269d" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansNetOfTax_49ed70a2-4dbd-43d5-8dc9-e561c1f436bb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansNetOfTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheetAbstract_8cf982ab-fe08-47e7-ae1b-f8b4fb19f05d" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansNetOfTax_49ed70a2-4dbd-43d5-8dc9-e561c1f436bb" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_DefinedBenefitPlanAmountsforAssetLiabilityandAccumulatedOtherComprehensiveIncomeRecognizedinStatementofFinancialPosition_29f484ba-ad29-4d83-ac8d-efc62b876666" xlink:href="ecvt-20231231.xsd#ecvt_DefinedBenefitPlanAmountsforAssetLiabilityandAccumulatedOtherComprehensiveIncomeRecognizedinStatementofFinancialPosition"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheetAbstract_8cf982ab-fe08-47e7-ae1b-f8b4fb19f05d" xlink:to="loc_ecvt_DefinedBenefitPlanAmountsforAssetLiabilityandAccumulatedOtherComprehensiveIncomeRecognizedinStatementofFinancialPosition_29f484ba-ad29-4d83-ac8d-efc62b876666" xlink:type="arc" order="4"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/BenefitPlansAccumulatedOtherComprehensiveIncomeDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#BenefitPlansAccumulatedOtherComprehensiveIncomeDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/BenefitPlansAccumulatedOtherComprehensiveIncomeDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_655f72fb-1c90-4ad2-845d-ebca9a7251e6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_97191687-0857-4921-a058-3ea8f204e3f0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_655f72fb-1c90-4ad2-845d-ebca9a7251e6" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_97191687-0857-4921-a058-3ea8f204e3f0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_1a1f9e16-fd2c-4b87-8a70-08b208914371" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_97191687-0857-4921-a058-3ea8f204e3f0" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_1a1f9e16-fd2c-4b87-8a70-08b208914371" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_1a1f9e16-fd2c-4b87-8a70-08b208914371_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_1a1f9e16-fd2c-4b87-8a70-08b208914371" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_1a1f9e16-fd2c-4b87-8a70-08b208914371_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_33554204-5841-4f3a-9bef-912c377e339f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_1a1f9e16-fd2c-4b87-8a70-08b208914371" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_33554204-5841-4f3a-9bef-912c377e339f" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PensionPlansDefinedBenefitMember_5cdffc31-b83f-47b0-828d-693b662bd0f7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PensionPlansDefinedBenefitMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_33554204-5841-4f3a-9bef-912c377e339f" xlink:to="loc_us-gaap_PensionPlansDefinedBenefitMember_5cdffc31-b83f-47b0-828d-693b662bd0f7" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_18394b42-f327-4410-89f6-c9e828b6b002" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_33554204-5841-4f3a-9bef-912c377e339f" xlink:to="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_18394b42-f327-4410-89f6-c9e828b6b002" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeLossAfterTaxAbstract_fda68f60-423a-4f8b-b9ba-aa5252ab1092" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeLossAfterTaxAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_655f72fb-1c90-4ad2-845d-ebca9a7251e6" xlink:to="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeLossAfterTaxAbstract_fda68f60-423a-4f8b-b9ba-aa5252ab1092" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetPriorServiceCostCreditBeforeTax_dde39bcd-270e-418a-91f7-a9043540b2c0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetPriorServiceCostCreditBeforeTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeLossAfterTaxAbstract_fda68f60-423a-4f8b-b9ba-aa5252ab1092" xlink:to="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetPriorServiceCostCreditBeforeTax_dde39bcd-270e-418a-91f7-a9043540b2c0" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetGainsLossesBeforeTax_677d590f-7c58-4323-b17c-d1035cd5f3ef" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetGainsLossesBeforeTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeLossAfterTaxAbstract_fda68f60-423a-4f8b-b9ba-aa5252ab1092" xlink:to="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetGainsLossesBeforeTax_677d590f-7c58-4323-b17c-d1035cd5f3ef" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTax_40296ed6-a8ba-46dc-8245-36dd4ffff8cc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeLossAfterTaxAbstract_fda68f60-423a-4f8b-b9ba-aa5252ab1092" xlink:to="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTax_40296ed6-a8ba-46dc-8245-36dd4ffff8cc" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPlanTax_8da7d11a-9be3-4cfc-aed3-84c5992bc45a" xlink:href="ecvt-20231231.xsd#ecvt_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPlanTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeLossAfterTaxAbstract_fda68f60-423a-4f8b-b9ba-aa5252ab1092" xlink:to="loc_ecvt_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPlanTax_8da7d11a-9be3-4cfc-aed3-84c5992bc45a" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansNetOfTax_5c07ad78-775e-4f52-8e66-90c423482902" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansNetOfTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeLossAfterTaxAbstract_fda68f60-423a-4f8b-b9ba-aa5252ab1092" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansNetOfTax_5c07ad78-775e-4f52-8e66-90c423482902" xlink:type="arc" order="5"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/BenefitPlansNetPeriodicBenefitCostDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#BenefitPlansNetPeriodicBenefitCostDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/BenefitPlansNetPeriodicBenefitCostDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_63584cff-4d1f-48b9-be0c-26d7a498994a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_8de31485-26c0-42c6-bc77-a74c583b9db8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_63584cff-4d1f-48b9-be0c-26d7a498994a" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_8de31485-26c0-42c6-bc77-a74c583b9db8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_c819e040-c97a-427a-9b91-31813892828e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_8de31485-26c0-42c6-bc77-a74c583b9db8" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_c819e040-c97a-427a-9b91-31813892828e" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_c819e040-c97a-427a-9b91-31813892828e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_c819e040-c97a-427a-9b91-31813892828e" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_c819e040-c97a-427a-9b91-31813892828e_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_19104545-1c58-4762-885d-f72c42e710a8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_c819e040-c97a-427a-9b91-31813892828e" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_19104545-1c58-4762-885d-f72c42e710a8" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PensionPlansDefinedBenefitMember_773bc933-b052-46b5-8cae-6d88127e2df6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PensionPlansDefinedBenefitMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_19104545-1c58-4762-885d-f72c42e710a8" xlink:to="loc_us-gaap_PensionPlansDefinedBenefitMember_773bc933-b052-46b5-8cae-6d88127e2df6" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_0808c029-00dd-440d-8bd2-1d11ecd2d1e2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_19104545-1c58-4762-885d-f72c42e710a8" xlink:to="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_0808c029-00dd-440d-8bd2-1d11ecd2d1e2" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_fe571954-7b42-485b-86da-319bba8104b3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_8de31485-26c0-42c6-bc77-a74c583b9db8" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_fe571954-7b42-485b-86da-319bba8104b3" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_fe571954-7b42-485b-86da-319bba8104b3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_fe571954-7b42-485b-86da-319bba8104b3" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_fe571954-7b42-485b-86da-319bba8104b3_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_8b9e72be-abfb-4e47-80d2-5917adf407ef" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_fe571954-7b42-485b-86da-319bba8104b3" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_8b9e72be-abfb-4e47-80d2-5917adf407ef" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticPlanMember_5282d085-c080-4534-ad01-33710b48cee6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DomesticPlanMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_8b9e72be-abfb-4e47-80d2-5917adf407ef" xlink:to="loc_us-gaap_DomesticPlanMember_5282d085-c080-4534-ad01-33710b48cee6" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_country_US_ef55ca0b-2a9f-4f65-a72f-ea1002efc8be" xlink:href="https://xbrl.sec.gov/country/2023/country-2023.xsd#country_US"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DomesticPlanMember_5282d085-c080-4534-ad01-33710b48cee6" xlink:to="loc_country_US_ef55ca0b-2a9f-4f65-a72f-ea1002efc8be" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignPlanMember_025dd429-384c-4ccb-be2d-a8c5ef506220" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ForeignPlanMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_8b9e72be-abfb-4e47-80d2-5917adf407ef" xlink:to="loc_us-gaap_ForeignPlanMember_025dd429-384c-4ccb-be2d-a8c5ef506220" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCostAbstract_6ec01fd1-ccaf-4fa9-92b8-eee10506a6bb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanNetPeriodicBenefitCostAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_63584cff-4d1f-48b9-be0c-26d7a498994a" xlink:to="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCostAbstract_6ec01fd1-ccaf-4fa9-92b8-eee10506a6bb" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanInterestCost_deebfe3b-300d-4a81-8e62-466354e6bae4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanInterestCost"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCostAbstract_6ec01fd1-ccaf-4fa9-92b8-eee10506a6bb" xlink:to="loc_us-gaap_DefinedBenefitPlanInterestCost_deebfe3b-300d-4a81-8e62-466354e6bae4" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit_0748de3e-c359-4687-90ed-7b6bf2de5999" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCostAbstract_6ec01fd1-ccaf-4fa9-92b8-eee10506a6bb" xlink:to="loc_us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit_0748de3e-c359-4687-90ed-7b6bf2de5999" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses_b98e8848-8140-4b70-912b-8911a6f4fc34" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCostAbstract_6ec01fd1-ccaf-4fa9-92b8-eee10506a6bb" xlink:to="loc_us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses_b98e8848-8140-4b70-912b-8911a6f4fc34" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets_1bfecbba-87c5-4698-a2e2-10f77acc0dc3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCostAbstract_6ec01fd1-ccaf-4fa9-92b8-eee10506a6bb" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets_1bfecbba-87c5-4698-a2e2-10f77acc0dc3" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1_62a7bbd3-5373-41dd-8264-bd9b86f75456" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCostAbstract_6ec01fd1-ccaf-4fa9-92b8-eee10506a6bb" xlink:to="loc_us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1_62a7bbd3-5373-41dd-8264-bd9b86f75456" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_489de7d0-8779-4f3c-aeb1-5e65b6ca3b7a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCostAbstract_6ec01fd1-ccaf-4fa9-92b8-eee10506a6bb" xlink:to="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_489de7d0-8779-4f3c-aeb1-5e65b6ca3b7a" xlink:type="arc" order="6"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/BenefitPlansAmountsrecognizedinNPBCandOCIDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#BenefitPlansAmountsrecognizedinNPBCandOCIDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/BenefitPlansAmountsrecognizedinNPBCandOCIDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_b21e679c-af62-4e25-86c0-4dccb96fce94" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_d733ea4e-607e-4351-9cbb-950e6cfd9508" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_b21e679c-af62-4e25-86c0-4dccb96fce94" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_d733ea4e-607e-4351-9cbb-950e6cfd9508" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_9600e699-18de-4f06-ba6e-966a5abecc33" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_d733ea4e-607e-4351-9cbb-950e6cfd9508" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_9600e699-18de-4f06-ba6e-966a5abecc33" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_9600e699-18de-4f06-ba6e-966a5abecc33_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_9600e699-18de-4f06-ba6e-966a5abecc33" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_9600e699-18de-4f06-ba6e-966a5abecc33_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_747b9e80-d0c6-4f4a-ac5e-5eadab85302f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_9600e699-18de-4f06-ba6e-966a5abecc33" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_747b9e80-d0c6-4f4a-ac5e-5eadab85302f" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PensionPlansDefinedBenefitMember_b2ee725e-9ebe-4e1b-90e2-669027f0217e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PensionPlansDefinedBenefitMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_747b9e80-d0c6-4f4a-ac5e-5eadab85302f" xlink:to="loc_us-gaap_PensionPlansDefinedBenefitMember_b2ee725e-9ebe-4e1b-90e2-669027f0217e" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_4241cab2-e6db-4f60-9ca0-91378127b62c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_747b9e80-d0c6-4f4a-ac5e-5eadab85302f" xlink:to="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_4241cab2-e6db-4f60-9ca0-91378127b62c" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax_74dd4425-8de1-43a0-9db1-ad496b5611ad" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_b21e679c-af62-4e25-86c0-4dccb96fce94" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax_74dd4425-8de1-43a0-9db1-ad496b5611ad" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax_e8b79687-ca27-4470-b16c-fb754899b36e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_b21e679c-af62-4e25-86c0-4dccb96fce94" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax_e8b79687-ca27-4470-b16c-fb754899b36e" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossFinalizationOfPensionAndNonPensionPostretirementPlanValuationBeforeTax_87edae22-0af8-452f-a22f-a427b30d8784" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossFinalizationOfPensionAndNonPensionPostretirementPlanValuationBeforeTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_b21e679c-af62-4e25-86c0-4dccb96fce94" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossFinalizationOfPensionAndNonPensionPostretirementPlanValuationBeforeTax_87edae22-0af8-452f-a22f-a427b30d8784" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeReclassificationAdjustmentsAndTax_096a650b-429e-4d9d-bed5-7bd17fa1de45" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeReclassificationAdjustmentsAndTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_b21e679c-af62-4e25-86c0-4dccb96fce94" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeReclassificationAdjustmentsAndTax_096a650b-429e-4d9d-bed5-7bd17fa1de45" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmountRecognizedInNetPeriodicBenefitCostAndOtherComprehensiveIncomeLossBeforeTax_6afce53c-bfe6-44e3-872c-194d60936dbf" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AmountRecognizedInNetPeriodicBenefitCostAndOtherComprehensiveIncomeLossBeforeTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_b21e679c-af62-4e25-86c0-4dccb96fce94" xlink:to="loc_us-gaap_AmountRecognizedInNetPeriodicBenefitCostAndOtherComprehensiveIncomeLossBeforeTax_6afce53c-bfe6-44e3-872c-194d60936dbf" xlink:type="arc" order="4"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/BenefitPlansProjectedandAccumulatedBenefitObligationDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#BenefitPlansProjectedandAccumulatedBenefitObligationDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/BenefitPlansProjectedandAccumulatedBenefitObligationDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_89622e5a-82fa-4321-a9f5-62b1f459b81b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_13942318-4511-41a1-8a26-99d589a34646" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_89622e5a-82fa-4321-a9f5-62b1f459b81b" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_13942318-4511-41a1-8a26-99d589a34646" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_f87c11d7-9bd9-4854-9e26-dbc4bb8c22da" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_13942318-4511-41a1-8a26-99d589a34646" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_f87c11d7-9bd9-4854-9e26-dbc4bb8c22da" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_f87c11d7-9bd9-4854-9e26-dbc4bb8c22da_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_f87c11d7-9bd9-4854-9e26-dbc4bb8c22da" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_f87c11d7-9bd9-4854-9e26-dbc4bb8c22da_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_68ad2c88-c8b0-4dbd-a54f-9c78b9d80961" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_f87c11d7-9bd9-4854-9e26-dbc4bb8c22da" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_68ad2c88-c8b0-4dbd-a54f-9c78b9d80961" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PensionPlansDefinedBenefitMember_36557239-82da-402a-aefc-52a509fe3cdb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PensionPlansDefinedBenefitMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_68ad2c88-c8b0-4dbd-a54f-9c78b9d80961" xlink:to="loc_us-gaap_PensionPlansDefinedBenefitMember_36557239-82da-402a-aefc-52a509fe3cdb" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanPensionPlanWithProjectedBenefitObligationInExcessOfPlanAssetsProjectedBenefitObligation_8e78ce2c-a928-40a0-ac5c-687c35160e0c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanPensionPlanWithProjectedBenefitObligationInExcessOfPlanAssetsProjectedBenefitObligation"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_89622e5a-82fa-4321-a9f5-62b1f459b81b" xlink:to="loc_us-gaap_DefinedBenefitPlanPensionPlanWithProjectedBenefitObligationInExcessOfPlanAssetsProjectedBenefitObligation_8e78ce2c-a928-40a0-ac5c-687c35160e0c" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateAccumulatedBenefitObligation_9af9d63a-0b67-4e4d-a382-9f17d579816b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateAccumulatedBenefitObligation"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_89622e5a-82fa-4321-a9f5-62b1f459b81b" xlink:to="loc_us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateAccumulatedBenefitObligation_9af9d63a-0b67-4e4d-a382-9f17d579816b" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_ca773b79-e3b1-4413-b2f0-643e78fa653c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanFairValueOfPlanAssets"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_89622e5a-82fa-4321-a9f5-62b1f459b81b" xlink:to="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_ca773b79-e3b1-4413-b2f0-643e78fa653c" xlink:type="arc" order="2"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/BenefitPlansWeightedAverageAssumptionsforPensionObligationDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#BenefitPlansWeightedAverageAssumptionsforPensionObligationDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/BenefitPlansWeightedAverageAssumptionsforPensionObligationDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_878d8e48-1ae0-47cc-aa53-2a6cde4418ee" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_24b967cc-ca5e-4886-9e57-96a18c1e9ff3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_878d8e48-1ae0-47cc-aa53-2a6cde4418ee" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_24b967cc-ca5e-4886-9e57-96a18c1e9ff3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_1ce9c4c8-405e-4172-8b9e-3481bccc6121" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_24b967cc-ca5e-4886-9e57-96a18c1e9ff3" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_1ce9c4c8-405e-4172-8b9e-3481bccc6121" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_1ce9c4c8-405e-4172-8b9e-3481bccc6121_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_1ce9c4c8-405e-4172-8b9e-3481bccc6121" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_1ce9c4c8-405e-4172-8b9e-3481bccc6121_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_290af6e2-e71d-4c72-81f6-ec49910c69d9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_1ce9c4c8-405e-4172-8b9e-3481bccc6121" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_290af6e2-e71d-4c72-81f6-ec49910c69d9" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PensionPlansDefinedBenefitMember_b6883304-0494-4750-a62d-4918ac8ef609" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PensionPlansDefinedBenefitMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_290af6e2-e71d-4c72-81f6-ec49910c69d9" xlink:to="loc_us-gaap_PensionPlansDefinedBenefitMember_b6883304-0494-4750-a62d-4918ac8ef609" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_017e68a4-71fc-4d43-b264-2e905ab4320c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_290af6e2-e71d-4c72-81f6-ec49910c69d9" xlink:to="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_017e68a4-71fc-4d43-b264-2e905ab4320c" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanWeightedAverageAssumptionsUsedInCalculatingBenefitObligationAbstract_3a2e7a75-1def-456a-b90b-21182d19ee16" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanWeightedAverageAssumptionsUsedInCalculatingBenefitObligationAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_878d8e48-1ae0-47cc-aa53-2a6cde4418ee" xlink:to="loc_us-gaap_DefinedBenefitPlanWeightedAverageAssumptionsUsedInCalculatingBenefitObligationAbstract_3a2e7a75-1def-456a-b90b-21182d19ee16" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate_0ca86d1b-531c-41d4-b7ca-284576e928a8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanWeightedAverageAssumptionsUsedInCalculatingBenefitObligationAbstract_3a2e7a75-1def-456a-b90b-21182d19ee16" xlink:to="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate_0ca86d1b-531c-41d4-b7ca-284576e928a8" xlink:type="arc" order="1"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/BenefitPlansWeightedAverageAssumptionsforNetPeriodicCostDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#BenefitPlansWeightedAverageAssumptionsforNetPeriodicCostDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/BenefitPlansWeightedAverageAssumptionsforNetPeriodicCostDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_d0c049b5-eb93-4a00-a827-838c796bcef5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_fa55f5dd-ffa9-4498-8724-0c4791911969" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_d0c049b5-eb93-4a00-a827-838c796bcef5" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_fa55f5dd-ffa9-4498-8724-0c4791911969" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_d7af0400-ea8c-474d-b4a3-bcd35819f514" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_fa55f5dd-ffa9-4498-8724-0c4791911969" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_d7af0400-ea8c-474d-b4a3-bcd35819f514" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_d7af0400-ea8c-474d-b4a3-bcd35819f514_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_d7af0400-ea8c-474d-b4a3-bcd35819f514" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_d7af0400-ea8c-474d-b4a3-bcd35819f514_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_9b7aff38-2b3c-4126-baf2-6833fbe17410" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_d7af0400-ea8c-474d-b4a3-bcd35819f514" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_9b7aff38-2b3c-4126-baf2-6833fbe17410" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PensionPlansDefinedBenefitMember_f44ee2ae-4273-4daa-908c-2cc6a7ab1cd3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PensionPlansDefinedBenefitMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_9b7aff38-2b3c-4126-baf2-6833fbe17410" xlink:to="loc_us-gaap_PensionPlansDefinedBenefitMember_f44ee2ae-4273-4daa-908c-2cc6a7ab1cd3" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_972d5a32-f24d-49c0-96f6-87416ee2344c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_9b7aff38-2b3c-4126-baf2-6833fbe17410" xlink:to="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_972d5a32-f24d-49c0-96f6-87416ee2344c" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_f1a636f4-90cf-4bf9-ad59-1abfac680708" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_fa55f5dd-ffa9-4498-8724-0c4791911969" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_f1a636f4-90cf-4bf9-ad59-1abfac680708" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_f1a636f4-90cf-4bf9-ad59-1abfac680708_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_f1a636f4-90cf-4bf9-ad59-1abfac680708" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_f1a636f4-90cf-4bf9-ad59-1abfac680708_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_76da1e0d-c970-438a-a550-71cf51c0ee1b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_f1a636f4-90cf-4bf9-ad59-1abfac680708" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_76da1e0d-c970-438a-a550-71cf51c0ee1b" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticPlanMember_4a28d00f-c5a7-4bf4-a159-823f79009c4b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DomesticPlanMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_76da1e0d-c970-438a-a550-71cf51c0ee1b" xlink:to="loc_us-gaap_DomesticPlanMember_4a28d00f-c5a7-4bf4-a159-823f79009c4b" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_country_US_74bb19cc-11f5-4199-8db5-62da6a03f019" xlink:href="https://xbrl.sec.gov/country/2023/country-2023.xsd#country_US"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DomesticPlanMember_4a28d00f-c5a7-4bf4-a159-823f79009c4b" xlink:to="loc_country_US_74bb19cc-11f5-4199-8db5-62da6a03f019" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignPlanMember_d59d65e4-f2c2-490d-8bfc-270e802cc072" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ForeignPlanMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_76da1e0d-c970-438a-a550-71cf51c0ee1b" xlink:to="loc_us-gaap_ForeignPlanMember_d59d65e4-f2c2-490d-8bfc-270e802cc072" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanWeightedAverageAssumptionsUsedInCalculatingNetPeriodicBenefitCostAbstract_24c1503c-a04c-4fc7-abf9-075efdcacda2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanWeightedAverageAssumptionsUsedInCalculatingNetPeriodicBenefitCostAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_d0c049b5-eb93-4a00-a827-838c796bcef5" xlink:to="loc_us-gaap_DefinedBenefitPlanWeightedAverageAssumptionsUsedInCalculatingNetPeriodicBenefitCostAbstract_24c1503c-a04c-4fc7-abf9-075efdcacda2" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate_ce69053d-f660-4e78-ab9a-83587da25cfd" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanWeightedAverageAssumptionsUsedInCalculatingNetPeriodicBenefitCostAbstract_24c1503c-a04c-4fc7-abf9-075efdcacda2" xlink:to="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate_ce69053d-f660-4e78-ab9a-83587da25cfd" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets_3b8a4631-77eb-4fb3-8b7c-a1638d1b2f3a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanWeightedAverageAssumptionsUsedInCalculatingNetPeriodicBenefitCostAbstract_24c1503c-a04c-4fc7-abf9-075efdcacda2" xlink:to="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets_3b8a4631-77eb-4fb3-8b7c-a1638d1b2f3a" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease_43729453-fee3-460e-8125-1e1c6970b71e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanWeightedAverageAssumptionsUsedInCalculatingNetPeriodicBenefitCostAbstract_24c1503c-a04c-4fc7-abf9-075efdcacda2" xlink:to="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease_43729453-fee3-460e-8125-1e1c6970b71e" xlink:type="arc" order="3"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/BenefitPlansPlanAssetsDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#BenefitPlansPlanAssetsDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/BenefitPlansPlanAssetsDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_6933753c-987d-4701-b2f6-1d675396f90c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_eac4a323-eb70-4a99-b567-30ea2d7c6e64" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_6933753c-987d-4701-b2f6-1d675396f90c" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_eac4a323-eb70-4a99-b567-30ea2d7c6e64" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_50c1047b-166b-42b8-a15f-19ede359208d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_eac4a323-eb70-4a99-b567-30ea2d7c6e64" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_50c1047b-166b-42b8-a15f-19ede359208d" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_50c1047b-166b-42b8-a15f-19ede359208d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_50c1047b-166b-42b8-a15f-19ede359208d" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_50c1047b-166b-42b8-a15f-19ede359208d_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_32c88a34-75b9-49d1-a3b1-3a3bd4f25c8d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_50c1047b-166b-42b8-a15f-19ede359208d" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_32c88a34-75b9-49d1-a3b1-3a3bd4f25c8d" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PensionPlansDefinedBenefitMember_14b78bb1-1cc0-4096-bbaf-5b9ca4b173f0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PensionPlansDefinedBenefitMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_32c88a34-75b9-49d1-a3b1-3a3bd4f25c8d" xlink:to="loc_us-gaap_PensionPlansDefinedBenefitMember_14b78bb1-1cc0-4096-bbaf-5b9ca4b173f0" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_be4c3851-c657-46f6-b447-b1f4fdc7bcfd" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_eac4a323-eb70-4a99-b567-30ea2d7c6e64" xlink:to="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_be4c3851-c657-46f6-b447-b1f4fdc7bcfd" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanAssetCategoriesDomain_be4c3851-c657-46f6-b447-b1f4fdc7bcfd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PlanAssetCategoriesDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_be4c3851-c657-46f6-b447-b1f4fdc7bcfd" xlink:to="loc_us-gaap_PlanAssetCategoriesDomain_be4c3851-c657-46f6-b447-b1f4fdc7bcfd_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanAssetCategoriesDomain_a1b7956f-9775-451c-8f04-28f89d5cf963" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PlanAssetCategoriesDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_be4c3851-c657-46f6-b447-b1f4fdc7bcfd" xlink:to="loc_us-gaap_PlanAssetCategoriesDomain_a1b7956f-9775-451c-8f04-28f89d5cf963" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsMember_7adcd965-6c16-47d3-8f0b-5c77d1512f48" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CashAndCashEquivalentsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_a1b7956f-9775-451c-8f04-28f89d5cf963" xlink:to="loc_us-gaap_CashAndCashEquivalentsMember_7adcd965-6c16-47d3-8f0b-5c77d1512f48" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesMember_275574cf-1433-4879-be5f-d4953ff5b15c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquitySecuritiesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_a1b7956f-9775-451c-8f04-28f89d5cf963" xlink:to="loc_us-gaap_EquitySecuritiesMember_275574cf-1433-4879-be5f-d4953ff5b15c" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrivateEquityFundsDomesticMember_97f48793-f064-496f-98e6-fd100d503494" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PrivateEquityFundsDomesticMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesMember_275574cf-1433-4879-be5f-d4953ff5b15c" xlink:to="loc_us-gaap_PrivateEquityFundsDomesticMember_97f48793-f064-496f-98e6-fd100d503494" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrivateEquityFundsForeignMember_f93d933a-5d3e-4b62-b40f-b3a6ecd0b130" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PrivateEquityFundsForeignMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesMember_275574cf-1433-4879-be5f-d4953ff5b15c" xlink:to="loc_us-gaap_PrivateEquityFundsForeignMember_f93d933a-5d3e-4b62-b40f-b3a6ecd0b130" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesMember_dbf612ea-9acc-4801-a231-432d418968db" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtSecuritiesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_a1b7956f-9775-451c-8f04-28f89d5cf963" xlink:to="loc_us-gaap_DebtSecuritiesMember_dbf612ea-9acc-4801-a231-432d418968db" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_4da5335b-2f44-4090-b6ca-d027ec237c7e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_USTreasuryAndGovernmentMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesMember_dbf612ea-9acc-4801-a231-432d418968db" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_4da5335b-2f44-4090-b6ca-d027ec237c7e" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_96f52bf8-f194-43c7-9079-b39ad6b57372" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CorporateDebtSecuritiesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesMember_dbf612ea-9acc-4801-a231-432d418968db" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_96f52bf8-f194-43c7-9079-b39ad6b57372" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_3ea9f007-1392-42ed-b6fc-800dee8e1b52" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_eac4a323-eb70-4a99-b567-30ea2d7c6e64" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_3ea9f007-1392-42ed-b6fc-800dee8e1b52" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_3ea9f007-1392-42ed-b6fc-800dee8e1b52_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_3ea9f007-1392-42ed-b6fc-800dee8e1b52" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_3ea9f007-1392-42ed-b6fc-800dee8e1b52_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_0462b8b0-af3e-497a-808a-6cfa3049b3bc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_3ea9f007-1392-42ed-b6fc-800dee8e1b52" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_0462b8b0-af3e-497a-808a-6cfa3049b3bc" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_6e4dbbdb-fe79-4ccb-b8ef-182da4080995" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueInputsLevel1Member"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_0462b8b0-af3e-497a-808a-6cfa3049b3bc" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_6e4dbbdb-fe79-4ccb-b8ef-182da4080995" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_7f2ec403-12fb-4374-a364-d62f82a7d4c1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueInputsLevel2Member"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_0462b8b0-af3e-497a-808a-6cfa3049b3bc" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_7f2ec403-12fb-4374-a364-d62f82a7d4c1" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_acca3007-bc21-42b5-b3a3-df10ed43ef2a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueInputsLevel3Member"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_0462b8b0-af3e-497a-808a-6cfa3049b3bc" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_acca3007-bc21-42b5-b3a3-df10ed43ef2a" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameAxis_fbb31a53-ccb1-44ae-9c81-d6198d9165ec" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanNameAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_eac4a323-eb70-4a99-b567-30ea2d7c6e64" xlink:to="loc_us-gaap_RetirementPlanNameAxis_fbb31a53-ccb1-44ae-9c81-d6198d9165ec" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameDomain_fbb31a53-ccb1-44ae-9c81-d6198d9165ec_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanNameAxis_fbb31a53-ccb1-44ae-9c81-d6198d9165ec" xlink:to="loc_us-gaap_RetirementPlanNameDomain_fbb31a53-ccb1-44ae-9c81-d6198d9165ec_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanNameDomain_5e78c2fa-c162-4b56-8231-df0981776281" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanNameAxis_fbb31a53-ccb1-44ae-9c81-d6198d9165ec" xlink:to="loc_us-gaap_RetirementPlanNameDomain_5e78c2fa-c162-4b56-8231-df0981776281" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_EcoServicesPensionEquityPlanMember_745068cc-8686-4bf7-af8c-d319d416ba85" xlink:href="ecvt-20231231.xsd#ecvt_EcoServicesPensionEquityPlanMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanNameDomain_5e78c2fa-c162-4b56-8231-df0981776281" xlink:to="loc_ecvt_EcoServicesPensionEquityPlanMember_745068cc-8686-4bf7-af8c-d319d416ba85" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_EcoServicesHourlyPensionPlanMember_895463f8-fac7-4608-927f-bf83f6d3f26e" xlink:href="ecvt-20231231.xsd#ecvt_EcoServicesHourlyPensionPlanMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanNameDomain_5e78c2fa-c162-4b56-8231-df0981776281" xlink:to="loc_ecvt_EcoServicesHourlyPensionPlanMember_895463f8-fac7-4608-927f-bf83f6d3f26e" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_467ec1fc-ee37-4b7f-9521-d13266ac2392" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanFairValueOfPlanAssets"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_6933753c-987d-4701-b2f6-1d675396f90c" xlink:to="loc_us-gaap_DefinedBenefitPlanFairValueOfPlanAssets_467ec1fc-ee37-4b7f-9521-d13266ac2392" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanPlanAssetsTargetAllocationPercentage_6e1d4fbb-f294-4f7a-ab1c-a92f0e149e7d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanPlanAssetsTargetAllocationPercentage"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_6933753c-987d-4701-b2f6-1d675396f90c" xlink:to="loc_us-gaap_DefinedBenefitPlanPlanAssetsTargetAllocationPercentage_6e1d4fbb-f294-4f7a-ab1c-a92f0e149e7d" xlink:type="arc" order="1"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/BenefitPlansBenefitPlanFuturePaymentsDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#BenefitPlansBenefitPlanFuturePaymentsDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/BenefitPlansBenefitPlanFuturePaymentsDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_f6c38363-0d7c-4ea9-9f86-396517151dcc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_b2de08f5-3896-4c64-bf79-2ccd6d14cf85" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_f6c38363-0d7c-4ea9-9f86-396517151dcc" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_b2de08f5-3896-4c64-bf79-2ccd6d14cf85" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_be3d4aa3-e33c-40cb-9217-3053a6fcb3f7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_b2de08f5-3896-4c64-bf79-2ccd6d14cf85" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_be3d4aa3-e33c-40cb-9217-3053a6fcb3f7" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_be3d4aa3-e33c-40cb-9217-3053a6fcb3f7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_be3d4aa3-e33c-40cb-9217-3053a6fcb3f7" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_be3d4aa3-e33c-40cb-9217-3053a6fcb3f7_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_5ff5ba4a-f76f-415c-abdb-5ac000453bd5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_be3d4aa3-e33c-40cb-9217-3053a6fcb3f7" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_5ff5ba4a-f76f-415c-abdb-5ac000453bd5" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PensionPlansDefinedBenefitMember_80b7255f-6e14-49e6-bc5f-3d4791e001f4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PensionPlansDefinedBenefitMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_5ff5ba4a-f76f-415c-abdb-5ac000453bd5" xlink:to="loc_us-gaap_PensionPlansDefinedBenefitMember_80b7255f-6e14-49e6-bc5f-3d4791e001f4" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_35487890-6090-4885-b4f2-7e3e2e531e7e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_5ff5ba4a-f76f-415c-abdb-5ac000453bd5" xlink:to="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_35487890-6090-4885-b4f2-7e3e2e531e7e" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanEstimatedFutureBenefitPaymentsAbstract_63a824b4-d95b-4956-af8a-ae89688c7c22" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanEstimatedFutureBenefitPaymentsAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_f6c38363-0d7c-4ea9-9f86-396517151dcc" xlink:to="loc_us-gaap_DefinedBenefitPlanEstimatedFutureBenefitPaymentsAbstract_63a824b4-d95b-4956-af8a-ae89688c7c22" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths_d9aafb13-9c4a-48ee-825f-b7716967cff1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanEstimatedFutureBenefitPaymentsAbstract_63a824b4-d95b-4956-af8a-ae89688c7c22" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths_d9aafb13-9c4a-48ee-825f-b7716967cff1" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo_6b075c60-79ab-4cd0-8001-54158866e9e7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanEstimatedFutureBenefitPaymentsAbstract_63a824b4-d95b-4956-af8a-ae89688c7c22" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo_6b075c60-79ab-4cd0-8001-54158866e9e7" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree_db6e4d2a-9511-46cf-b6a0-afc9881abce5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanEstimatedFutureBenefitPaymentsAbstract_63a824b4-d95b-4956-af8a-ae89688c7c22" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree_db6e4d2a-9511-46cf-b6a0-afc9881abce5" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour_aa696eaf-d771-4c48-887f-c83291aeea69" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanEstimatedFutureBenefitPaymentsAbstract_63a824b4-d95b-4956-af8a-ae89688c7c22" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour_aa696eaf-d771-4c48-887f-c83291aeea69" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive_06376c46-bb9d-4f77-8220-6f6c6302467a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanEstimatedFutureBenefitPaymentsAbstract_63a824b4-d95b-4956-af8a-ae89688c7c22" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive_06376c46-bb9d-4f77-8220-6f6c6302467a" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter_d50ecbd5-ac94-4b19-8f0b-e9287f569658" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanEstimatedFutureBenefitPaymentsAbstract_63a824b4-d95b-4956-af8a-ae89688c7c22" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter_d50ecbd5-ac94-4b19-8f0b-e9287f569658" xlink:type="arc" order="6"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedFutureEmployerContributionsNextFiscalYear_a8211b57-1a1e-4936-bfeb-4e76b626932c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanExpectedFutureEmployerContributionsNextFiscalYear"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanEstimatedFutureBenefitPaymentsAbstract_63a824b4-d95b-4956-af8a-ae89688c7c22" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedFutureEmployerContributionsNextFiscalYear_a8211b57-1a1e-4936-bfeb-4e76b626932c" xlink:type="arc" order="7"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/StockBasedCompensationNarrativesDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#StockBasedCompensationNarrativesDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/StockBasedCompensationNarrativesDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ada3bdbd-60ae-4f4b-af1d-3b6413600071" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_26cae2b6-b9e0-432a-baa8-10c0139c56cf" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ada3bdbd-60ae-4f4b-af1d-3b6413600071" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_26cae2b6-b9e0-432a-baa8-10c0139c56cf" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_80476c52-067f-4e5f-8448-a207f952b2c6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PlanNameAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_26cae2b6-b9e0-432a-baa8-10c0139c56cf" xlink:to="loc_us-gaap_PlanNameAxis_80476c52-067f-4e5f-8448-a207f952b2c6" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_80476c52-067f-4e5f-8448-a207f952b2c6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PlanNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_80476c52-067f-4e5f-8448-a207f952b2c6" xlink:to="loc_us-gaap_PlanNameDomain_80476c52-067f-4e5f-8448-a207f952b2c6_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_5f3ea3be-f391-432d-822f-60ccfb1d2399" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PlanNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_80476c52-067f-4e5f-8448-a207f952b2c6" xlink:to="loc_us-gaap_PlanNameDomain_5f3ea3be-f391-432d-822f-60ccfb1d2399" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_TwoThousandSeventeenOmnibusPlanMemberMember_3146c71b-6a37-45bb-a801-77b732471fea" xlink:href="ecvt-20231231.xsd#ecvt_TwoThousandSeventeenOmnibusPlanMemberMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_5f3ea3be-f391-432d-822f-60ccfb1d2399" xlink:to="loc_ecvt_TwoThousandSeventeenOmnibusPlanMemberMember_3146c71b-6a37-45bb-a801-77b732471fea" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationAxis_fd099592-cff8-4fad-b322-db664d4ab674" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupClassificationAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_26cae2b6-b9e0-432a-baa8-10c0139c56cf" xlink:to="loc_us-gaap_DisposalGroupClassificationAxis_fd099592-cff8-4fad-b322-db664d4ab674" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_fd099592-cff8-4fad-b322-db664d4ab674_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupClassificationDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_fd099592-cff8-4fad-b322-db664d4ab674" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_fd099592-cff8-4fad-b322-db664d4ab674_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_4aa13fdc-8353-4b13-b86c-ba5ffcc5aa17" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupClassificationDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_fd099592-cff8-4fad-b322-db664d4ab674" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_4aa13fdc-8353-4b13-b86c-ba5ffcc5aa17" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DiscontinuedOperationsDisposedOfBySaleMember_c1f5d51b-8986-4c6e-97de-691296e1a8e6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DiscontinuedOperationsDisposedOfBySaleMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupClassificationDomain_4aa13fdc-8353-4b13-b86c-ba5ffcc5aa17" xlink:to="loc_us-gaap_DiscontinuedOperationsDisposedOfBySaleMember_c1f5d51b-8986-4c6e-97de-691296e1a8e6" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_b26bb3b3-f833-4f53-94ce-e0b159c52e1b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AwardTypeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_26cae2b6-b9e0-432a-baa8-10c0139c56cf" xlink:to="loc_us-gaap_AwardTypeAxis_b26bb3b3-f833-4f53-94ce-e0b159c52e1b" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_b26bb3b3-f833-4f53-94ce-e0b159c52e1b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_b26bb3b3-f833-4f53-94ce-e0b159c52e1b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_b26bb3b3-f833-4f53-94ce-e0b159c52e1b_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d1c1bc3f-d2e1-493b-8db4-4375e794174f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_b26bb3b3-f833-4f53-94ce-e0b159c52e1b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d1c1bc3f-d2e1-493b-8db4-4375e794174f" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_27b17d11-e001-40bc-85b9-7214f4b58725" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EmployeeStockOptionMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d1c1bc3f-d2e1-493b-8db4-4375e794174f" xlink:to="loc_us-gaap_EmployeeStockOptionMember_27b17d11-e001-40bc-85b9-7214f4b58725" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PerformanceStockUnitsMember_c0dd5876-6a74-4343-8403-5accf06dee86" xlink:href="ecvt-20231231.xsd#ecvt_PerformanceStockUnitsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d1c1bc3f-d2e1-493b-8db4-4375e794174f" xlink:to="loc_ecvt_PerformanceStockUnitsMember_c0dd5876-6a74-4343-8403-5accf06dee86" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PerformanceMeasureAxis_6ef249f7-b968-4a67-8604-7c59b54c54b9" xlink:href="ecvt-20231231.xsd#ecvt_PerformanceMeasureAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_26cae2b6-b9e0-432a-baa8-10c0139c56cf" xlink:to="loc_ecvt_PerformanceMeasureAxis_6ef249f7-b968-4a67-8604-7c59b54c54b9" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PerformanceMeasureDomain_6ef249f7-b968-4a67-8604-7c59b54c54b9_default" xlink:href="ecvt-20231231.xsd#ecvt_PerformanceMeasureDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_ecvt_PerformanceMeasureAxis_6ef249f7-b968-4a67-8604-7c59b54c54b9" xlink:to="loc_ecvt_PerformanceMeasureDomain_6ef249f7-b968-4a67-8604-7c59b54c54b9_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PerformanceMeasureDomain_6a3719e6-a4b8-4f2d-a92c-d3514c3b0ad3" xlink:href="ecvt-20231231.xsd#ecvt_PerformanceMeasureDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ecvt_PerformanceMeasureAxis_6ef249f7-b968-4a67-8604-7c59b54c54b9" xlink:to="loc_ecvt_PerformanceMeasureDomain_6a3719e6-a4b8-4f2d-a92c-d3514c3b0ad3" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_CompanyPerformanceMeasureMember_3b8aac5d-7959-477f-aad4-1153a0a6c0e2" xlink:href="ecvt-20231231.xsd#ecvt_CompanyPerformanceMeasureMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecvt_PerformanceMeasureDomain_6a3719e6-a4b8-4f2d-a92c-d3514c3b0ad3" xlink:to="loc_ecvt_CompanyPerformanceMeasureMember_3b8aac5d-7959-477f-aad4-1153a0a6c0e2" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_TotalShareholderReturnTSRMember_5ce94102-f197-4ac3-bb08-57f0c22085af" xlink:href="ecvt-20231231.xsd#ecvt_TotalShareholderReturnTSRMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecvt_PerformanceMeasureDomain_6a3719e6-a4b8-4f2d-a92c-d3514c3b0ad3" xlink:to="loc_ecvt_TotalShareholderReturnTSRMember_5ce94102-f197-4ac3-bb08-57f0c22085af" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateAxis_b8c8565d-49ee-4bf6-b071-70a14631bd37" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AwardDateAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_26cae2b6-b9e0-432a-baa8-10c0139c56cf" xlink:to="loc_us-gaap_AwardDateAxis_b8c8565d-49ee-4bf6-b071-70a14631bd37" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateDomain_b8c8565d-49ee-4bf6-b071-70a14631bd37_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AwardDateDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardDateAxis_b8c8565d-49ee-4bf6-b071-70a14631bd37" xlink:to="loc_us-gaap_AwardDateDomain_b8c8565d-49ee-4bf6-b071-70a14631bd37_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateDomain_5ff3bc2e-a2df-480c-8ebd-4d8843944049" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AwardDateDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardDateAxis_b8c8565d-49ee-4bf6-b071-70a14631bd37" xlink:to="loc_us-gaap_AwardDateDomain_5ff3bc2e-a2df-480c-8ebd-4d8843944049" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_A2023GrantsMember_d624d6fb-49e5-4a8f-a798-d0b4888e4b1d" xlink:href="ecvt-20231231.xsd#ecvt_A2023GrantsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_5ff3bc2e-a2df-480c-8ebd-4d8843944049" xlink:to="loc_ecvt_A2023GrantsMember_d624d6fb-49e5-4a8f-a798-d0b4888e4b1d" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_A2022GrantsMember_d6d08aae-80cf-48b7-8241-89bb6afd93da" xlink:href="ecvt-20231231.xsd#ecvt_A2022GrantsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_5ff3bc2e-a2df-480c-8ebd-4d8843944049" xlink:to="loc_ecvt_A2022GrantsMember_d6d08aae-80cf-48b7-8241-89bb6afd93da" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_A2021GrantsMember_a99b8aed-3ac3-4d39-ba20-6494da34d844" xlink:href="ecvt-20231231.xsd#ecvt_A2021GrantsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_5ff3bc2e-a2df-480c-8ebd-4d8843944049" xlink:to="loc_ecvt_A2021GrantsMember_a99b8aed-3ac3-4d39-ba20-6494da34d844" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_A2020GrantsMember_7e73b2b9-c40f-4e9c-8e8c-9849cbd373c4" xlink:href="ecvt-20231231.xsd#ecvt_A2020GrantsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_5ff3bc2e-a2df-480c-8ebd-4d8843944049" xlink:to="loc_ecvt_A2020GrantsMember_7e73b2b9-c40f-4e9c-8e8c-9849cbd373c4" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_5f1d56f5-266c-4160-b842-590f65d1d8b2" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_26cae2b6-b9e0-432a-baa8-10c0139c56cf" xlink:to="loc_srt_RangeAxis_5f1d56f5-266c-4160-b842-590f65d1d8b2" xlink:type="arc" order="6"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_5f1d56f5-266c-4160-b842-590f65d1d8b2_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_5f1d56f5-266c-4160-b842-590f65d1d8b2" xlink:to="loc_srt_RangeMember_5f1d56f5-266c-4160-b842-590f65d1d8b2_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_05e16fe7-b605-475b-8370-ecfcc257c6cd" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_5f1d56f5-266c-4160-b842-590f65d1d8b2" xlink:to="loc_srt_RangeMember_05e16fe7-b605-475b-8370-ecfcc257c6cd" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_3261d84e-3c98-4b41-a17d-f59e7b8a4f85" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MinimumMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_05e16fe7-b605-475b-8370-ecfcc257c6cd" xlink:to="loc_srt_MinimumMember_3261d84e-3c98-4b41-a17d-f59e7b8a4f85" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_3a0133f3-4072-435a-95a6-2c09d77e298a" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MaximumMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_05e16fe7-b605-475b-8370-ecfcc257c6cd" xlink:to="loc_srt_MaximumMember_3a0133f3-4072-435a-95a6-2c09d77e298a" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_5abce853-c366-4601-98c1-e7c8edf73b01" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ada3bdbd-60ae-4f4b-af1d-3b6413600071" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_5abce853-c366-4601-98c1-e7c8edf73b01" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockDividendsPerShareCashPaid_072bec5f-2cbe-44e6-9d7d-7fa6b049498d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockDividendsPerShareCashPaid"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ada3bdbd-60ae-4f4b-af1d-3b6413600071" xlink:to="loc_us-gaap_CommonStockDividendsPerShareCashPaid_072bec5f-2cbe-44e6-9d7d-7fa6b049498d" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost_12fc1386-5616-4046-9d7e-963412526b8f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ada3bdbd-60ae-4f4b-af1d-3b6413600071" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost_12fc1386-5616-4046-9d7e-963412526b8f" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_bbd96136-0064-4744-ac7b-91184a0501e0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ada3bdbd-60ae-4f4b-af1d-3b6413600071" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_bbd96136-0064-4744-ac7b-91184a0501e0" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_dccbee0c-8c3a-436d-b734-bad6ee3c2cea" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ada3bdbd-60ae-4f4b-af1d-3b6413600071" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_dccbee0c-8c3a-436d-b734-bad6ee3c2cea" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromStockOptionsExercised_7dc48650-a61c-4c2e-9a74-e62614d193c8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ProceedsFromStockOptionsExercised"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ada3bdbd-60ae-4f4b-af1d-3b6413600071" xlink:to="loc_us-gaap_ProceedsFromStockOptionsExercised_7dc48650-a61c-4c2e-9a74-e62614d193c8" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_553f6f3f-ad1b-4d5e-9d37-900c6011383f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ada3bdbd-60ae-4f4b-af1d-3b6413600071" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_553f6f3f-ad1b-4d5e-9d37-900c6011383f" xlink:type="arc" order="6"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1_24526042-936a-40e1-be86-75f53d6ce854" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ada3bdbd-60ae-4f4b-af1d-3b6413600071" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1_24526042-936a-40e1-be86-75f53d6ce854" xlink:type="arc" order="7"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_65ffa7f3-edf0-435c-9930-e0d32928193c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ada3bdbd-60ae-4f4b-af1d-3b6413600071" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_65ffa7f3-edf0-435c-9930-e0d32928193c" xlink:type="arc" order="8"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_c5c03a37-bade-495e-a70d-a4d81586cfae" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ada3bdbd-60ae-4f4b-af1d-3b6413600071" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_c5c03a37-bade-495e-a70d-a4d81586cfae" xlink:type="arc" order="9"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PerformanceStockUnitsGrantsInPeriodWeightedAverageGrantDateFairValue_640e9e38-f0e9-474b-8948-245fc26d6966" xlink:href="ecvt-20231231.xsd#ecvt_PerformanceStockUnitsGrantsInPeriodWeightedAverageGrantDateFairValue"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ada3bdbd-60ae-4f4b-af1d-3b6413600071" xlink:to="loc_ecvt_PerformanceStockUnitsGrantsInPeriodWeightedAverageGrantDateFairValue_640e9e38-f0e9-474b-8948-245fc26d6966" xlink:type="arc" order="10"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodPercentage_f961e854-9453-4c9b-b40c-4f9f26f17834" xlink:href="ecvt-20231231.xsd#ecvt_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodPercentage"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ada3bdbd-60ae-4f4b-af1d-3b6413600071" xlink:to="loc_ecvt_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodPercentage_f961e854-9453-4c9b-b40c-4f9f26f17834" xlink:type="arc" order="11"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/StockBasedCompensationStockOptionActivityDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#StockBasedCompensationStockOptionActivityDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/StockBasedCompensationStockOptionActivityDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_634e152f-89cb-44a7-a283-f1f489412c88" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_88c70dce-7e45-4e9f-b444-74f45b1e7c34" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_634e152f-89cb-44a7-a283-f1f489412c88" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_88c70dce-7e45-4e9f-b444-74f45b1e7c34" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_071ffb01-dc20-4027-9120-1ca912e39885" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AwardTypeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_88c70dce-7e45-4e9f-b444-74f45b1e7c34" xlink:to="loc_us-gaap_AwardTypeAxis_071ffb01-dc20-4027-9120-1ca912e39885" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_071ffb01-dc20-4027-9120-1ca912e39885_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_071ffb01-dc20-4027-9120-1ca912e39885" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_071ffb01-dc20-4027-9120-1ca912e39885_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d47d360e-1428-4212-ab03-7be75221aaae" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_071ffb01-dc20-4027-9120-1ca912e39885" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d47d360e-1428-4212-ab03-7be75221aaae" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_4729bc23-c3e0-47e0-9b6f-8961e6ae6796" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EmployeeStockOptionMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d47d360e-1428-4212-ab03-7be75221aaae" xlink:to="loc_us-gaap_EmployeeStockOptionMember_4729bc23-c3e0-47e0-9b6f-8961e6ae6796" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_747e2c3d-c072-4a1f-9472-de58fc0201ca" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_634e152f-89cb-44a7-a283-f1f489412c88" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_747e2c3d-c072-4a1f-9472-de58fc0201ca" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_64fdba6d-15e9-4972-b636-7d2003b732dc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_747e2c3d-c072-4a1f-9472-de58fc0201ca" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_64fdba6d-15e9-4972-b636-7d2003b732dc" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_d5b27ef5-8b76-4cd2-a920-a3b031d61003" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_747e2c3d-c072-4a1f-9472-de58fc0201ca" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_d5b27ef5-8b76-4cd2-a920-a3b031d61003" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod_97e4113e-ab54-4a32-a57e-f40f933815f9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_747e2c3d-c072-4a1f-9472-de58fc0201ca" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod_97e4113e-ab54-4a32-a57e-f40f933815f9" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod_5c0e8fc3-daae-4184-9407-a8de8f5b8b60" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_747e2c3d-c072-4a1f-9472-de58fc0201ca" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod_5c0e8fc3-daae-4184-9407-a8de8f5b8b60" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_a4caa190-363a-4278-b9d2-340033f02d56" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_32ffeb72-94f9-4ce4-8017-3ab6e26c6da5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_747e2c3d-c072-4a1f-9472-de58fc0201ca" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_32ffeb72-94f9-4ce4-8017-3ab6e26c6da5" xlink:type="arc" order="6"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_829664d6-a35e-4369-80a7-06d8fce82bd9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_634e152f-89cb-44a7-a283-f1f489412c88" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_829664d6-a35e-4369-80a7-06d8fce82bd9" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_84656d7c-e36f-47a8-8b24-945019a26ec1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_829664d6-a35e-4369-80a7-06d8fce82bd9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_84656d7c-e36f-47a8-8b24-945019a26ec1" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_ddfc52d3-0e73-40fb-ab50-da5f1360bb68" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_829664d6-a35e-4369-80a7-06d8fce82bd9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_ddfc52d3-0e73-40fb-ab50-da5f1360bb68" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice_6406cf5a-cd36-4378-9b08-23e9960ed4b6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_829664d6-a35e-4369-80a7-06d8fce82bd9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice_6406cf5a-cd36-4378-9b08-23e9960ed4b6" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice_13e333db-a1a0-4570-b5b8-bcec96daf482" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_829664d6-a35e-4369-80a7-06d8fce82bd9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice_13e333db-a1a0-4570-b5b8-bcec96daf482" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_738979e1-6252-44da-a93e-3e1798b8066a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_9f46e1f1-1374-47f9-89e3-34a65456644e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_829664d6-a35e-4369-80a7-06d8fce82bd9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_9f46e1f1-1374-47f9-89e3-34a65456644e" xlink:type="arc" order="6"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_a977bb32-12d7-4989-9b25-6a003dd45bb5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_634e152f-89cb-44a7-a283-f1f489412c88" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_a977bb32-12d7-4989-9b25-6a003dd45bb5" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_710f01fe-7892-4cd0-80a8-b38b2cfa7456" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_a977bb32-12d7-4989-9b25-6a003dd45bb5" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_710f01fe-7892-4cd0-80a8-b38b2cfa7456" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_1d95eff8-359a-46c5-aac8-128196a16c10" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_a977bb32-12d7-4989-9b25-6a003dd45bb5" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_1d95eff8-359a-46c5-aac8-128196a16c10" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_a2f01e7a-9fc1-4bb6-a2e6-3b2b82b6cc7a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_a977bb32-12d7-4989-9b25-6a003dd45bb5" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_a2f01e7a-9fc1-4bb6-a2e6-3b2b82b6cc7a" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_5267d63f-1db3-4d1b-bbb7-b74fa31ca47b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_a977bb32-12d7-4989-9b25-6a003dd45bb5" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_5267d63f-1db3-4d1b-bbb7-b74fa31ca47b" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockDividendsPerShareCashPaid_7e229899-8ac7-4db7-a40f-cc661df25eee" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockDividendsPerShareCashPaid"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_634e152f-89cb-44a7-a283-f1f489412c88" xlink:to="loc_us-gaap_CommonStockDividendsPerShareCashPaid_7e229899-8ac7-4db7-a40f-cc661df25eee" xlink:type="arc" order="3"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/StockBasedCompensationRestrictedStockAwardsRestrictedStockUnitsandPerformanceStockUnitsDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#StockBasedCompensationRestrictedStockAwardsRestrictedStockUnitsandPerformanceStockUnitsDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/StockBasedCompensationRestrictedStockAwardsRestrictedStockUnitsandPerformanceStockUnitsDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_eacd0824-48aa-4e75-8cf7-ab7faa6ea1ee" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_7526955b-c1f8-4d7d-9a18-ba1f4628fb55" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_eacd0824-48aa-4e75-8cf7-ab7faa6ea1ee" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_7526955b-c1f8-4d7d-9a18-ba1f4628fb55" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_0ef3e035-4890-4231-8139-fc24f6860628" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AwardTypeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_7526955b-c1f8-4d7d-9a18-ba1f4628fb55" xlink:to="loc_us-gaap_AwardTypeAxis_0ef3e035-4890-4231-8139-fc24f6860628" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0ef3e035-4890-4231-8139-fc24f6860628_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_0ef3e035-4890-4231-8139-fc24f6860628" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0ef3e035-4890-4231-8139-fc24f6860628_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c63cb2ae-f752-4dfb-9787-da760fbc6620" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_0ef3e035-4890-4231-8139-fc24f6860628" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c63cb2ae-f752-4dfb-9787-da760fbc6620" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_bc51c1f0-adf3-497b-afd2-2c792393b5ad" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestrictedStockMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c63cb2ae-f752-4dfb-9787-da760fbc6620" xlink:to="loc_us-gaap_RestrictedStockMember_bc51c1f0-adf3-497b-afd2-2c792393b5ad" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_96e247c8-2b99-4511-87ef-fa8a69794a49" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c63cb2ae-f752-4dfb-9787-da760fbc6620" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_96e247c8-2b99-4511-87ef-fa8a69794a49" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PerformanceStockUnitsMember_73a04310-9325-47bb-bb72-0a0454efed54" xlink:href="ecvt-20231231.xsd#ecvt_PerformanceStockUnitsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c63cb2ae-f752-4dfb-9787-da760fbc6620" xlink:to="loc_ecvt_PerformanceStockUnitsMember_73a04310-9325-47bb-bb72-0a0454efed54" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_79e8a535-f5ec-4a76-9b73-ce769bbeea57" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PerformanceSharesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c63cb2ae-f752-4dfb-9787-da760fbc6620" xlink:to="loc_us-gaap_PerformanceSharesMember_79e8a535-f5ec-4a76-9b73-ce769bbeea57" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueAxis_855b271e-22bb-4fce-a0ff-b96cffe0d2e0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ValuationTechniqueAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_7526955b-c1f8-4d7d-9a18-ba1f4628fb55" xlink:to="loc_us-gaap_ValuationTechniqueAxis_855b271e-22bb-4fce-a0ff-b96cffe0d2e0" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueDomain_855b271e-22bb-4fce-a0ff-b96cffe0d2e0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ValuationTechniqueDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ValuationTechniqueAxis_855b271e-22bb-4fce-a0ff-b96cffe0d2e0" xlink:to="loc_us-gaap_ValuationTechniqueDomain_855b271e-22bb-4fce-a0ff-b96cffe0d2e0_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueDomain_aedd870f-c804-4d77-a16c-43d6eeb6fe8f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ValuationTechniqueDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ValuationTechniqueAxis_855b271e-22bb-4fce-a0ff-b96cffe0d2e0" xlink:to="loc_us-gaap_ValuationTechniqueDomain_aedd870f-c804-4d77-a16c-43d6eeb6fe8f" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_A2020GrantsMember_5ab9c2f9-4462-4631-b0c8-7a2d6ba54fac" xlink:href="ecvt-20231231.xsd#ecvt_A2020GrantsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationTechniqueDomain_aedd870f-c804-4d77-a16c-43d6eeb6fe8f" xlink:to="loc_ecvt_A2020GrantsMember_5ab9c2f9-4462-4631-b0c8-7a2d6ba54fac" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_A2021GrantsMember_cb0007d1-73bf-4102-a2da-e671477d3127" xlink:href="ecvt-20231231.xsd#ecvt_A2021GrantsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationTechniqueDomain_aedd870f-c804-4d77-a16c-43d6eeb6fe8f" xlink:to="loc_ecvt_A2021GrantsMember_cb0007d1-73bf-4102-a2da-e671477d3127" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_A2022GrantsMember_b93bc843-44bf-4d45-a074-242f9ef40aa1" xlink:href="ecvt-20231231.xsd#ecvt_A2022GrantsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationTechniqueDomain_aedd870f-c804-4d77-a16c-43d6eeb6fe8f" xlink:to="loc_ecvt_A2022GrantsMember_b93bc843-44bf-4d45-a074-242f9ef40aa1" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_A2023GrantsMember_71de0c88-bda7-4cdd-88e6-3d7b51569272" xlink:href="ecvt-20231231.xsd#ecvt_A2023GrantsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationTechniqueDomain_aedd870f-c804-4d77-a16c-43d6eeb6fe8f" xlink:to="loc_ecvt_A2023GrantsMember_71de0c88-bda7-4cdd-88e6-3d7b51569272" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateAxis_c29abf48-8d2f-4c2b-9ec8-15da3a4bcf8a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AwardDateAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_7526955b-c1f8-4d7d-9a18-ba1f4628fb55" xlink:to="loc_us-gaap_AwardDateAxis_c29abf48-8d2f-4c2b-9ec8-15da3a4bcf8a" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateDomain_c29abf48-8d2f-4c2b-9ec8-15da3a4bcf8a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AwardDateDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardDateAxis_c29abf48-8d2f-4c2b-9ec8-15da3a4bcf8a" xlink:to="loc_us-gaap_AwardDateDomain_c29abf48-8d2f-4c2b-9ec8-15da3a4bcf8a_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateDomain_33432757-cae5-4fc1-b40c-40704e54acf7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AwardDateDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardDateAxis_c29abf48-8d2f-4c2b-9ec8-15da3a4bcf8a" xlink:to="loc_us-gaap_AwardDateDomain_33432757-cae5-4fc1-b40c-40704e54acf7" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_A2020GrantsMember_d4331507-c919-467c-a5ab-a275fefcb738" xlink:href="ecvt-20231231.xsd#ecvt_A2020GrantsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_33432757-cae5-4fc1-b40c-40704e54acf7" xlink:to="loc_ecvt_A2020GrantsMember_d4331507-c919-467c-a5ab-a275fefcb738" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_A2021GrantsMember_5f1a10fc-68ee-4cfa-96f8-bd88ea3472f1" xlink:href="ecvt-20231231.xsd#ecvt_A2021GrantsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_33432757-cae5-4fc1-b40c-40704e54acf7" xlink:to="loc_ecvt_A2021GrantsMember_5f1a10fc-68ee-4cfa-96f8-bd88ea3472f1" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_A2022GrantsMember_5085266d-d427-4919-ae16-822d6953ec84" xlink:href="ecvt-20231231.xsd#ecvt_A2022GrantsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_33432757-cae5-4fc1-b40c-40704e54acf7" xlink:to="loc_ecvt_A2022GrantsMember_5085266d-d427-4919-ae16-822d6953ec84" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_A2023GrantsMember_9ad27783-fc43-48da-9dbc-7c77887ff467" xlink:href="ecvt-20231231.xsd#ecvt_A2023GrantsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_33432757-cae5-4fc1-b40c-40704e54acf7" xlink:to="loc_ecvt_A2023GrantsMember_9ad27783-fc43-48da-9dbc-7c77887ff467" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualAxis_ad84ed70-b189-42c7-97fc-ea0c4ccf1fcb" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_TitleOfIndividualAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_7526955b-c1f8-4d7d-9a18-ba1f4628fb55" xlink:to="loc_srt_TitleOfIndividualAxis_ad84ed70-b189-42c7-97fc-ea0c4ccf1fcb" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_ad84ed70-b189-42c7-97fc-ea0c4ccf1fcb_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_TitleOfIndividualAxis_ad84ed70-b189-42c7-97fc-ea0c4ccf1fcb" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_ad84ed70-b189-42c7-97fc-ea0c4ccf1fcb_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_c5136b3d-d7a4-4774-89cb-f029bf81168c" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_TitleOfIndividualAxis_ad84ed70-b189-42c7-97fc-ea0c4ccf1fcb" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_c5136b3d-d7a4-4774-89cb-f029bf81168c" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_DirectorMember_2367866f-d215-4955-a817-858dfed192b6" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_DirectorMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_c5136b3d-d7a4-4774-89cb-f029bf81168c" xlink:to="loc_srt_DirectorMember_2367866f-d215-4955-a817-858dfed192b6" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_EmployeeMember_aa9cac4f-7e76-491b-acc0-c70c94c8857e" xlink:href="ecvt-20231231.xsd#ecvt_EmployeeMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_c5136b3d-d7a4-4774-89cb-f029bf81168c" xlink:to="loc_ecvt_EmployeeMember_aa9cac4f-7e76-491b-acc0-c70c94c8857e" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_d6e401a0-acb2-487f-b4d0-4ad42fdb642f" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_7526955b-c1f8-4d7d-9a18-ba1f4628fb55" xlink:to="loc_srt_RangeAxis_d6e401a0-acb2-487f-b4d0-4ad42fdb642f" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_d6e401a0-acb2-487f-b4d0-4ad42fdb642f_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_d6e401a0-acb2-487f-b4d0-4ad42fdb642f" xlink:to="loc_srt_RangeMember_d6e401a0-acb2-487f-b4d0-4ad42fdb642f_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_0aaca218-7d9a-41a4-b0d3-c1e28e0355f5" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_d6e401a0-acb2-487f-b4d0-4ad42fdb642f" xlink:to="loc_srt_RangeMember_0aaca218-7d9a-41a4-b0d3-c1e28e0355f5" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_d32e7310-00eb-4378-bbcf-e14eb3c631b3" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MinimumMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_0aaca218-7d9a-41a4-b0d3-c1e28e0355f5" xlink:to="loc_srt_MinimumMember_d32e7310-00eb-4378-bbcf-e14eb3c631b3" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_5783816a-1014-4636-8294-6dd1ff4c7ccc" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MaximumMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_0aaca218-7d9a-41a4-b0d3-c1e28e0355f5" xlink:to="loc_srt_MaximumMember_5783816a-1014-4636-8294-6dd1ff4c7ccc" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_38a074a2-03d6-4cd9-88fc-27cfe81c8262" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_eacd0824-48aa-4e75-8cf7-ab7faa6ea1ee" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_38a074a2-03d6-4cd9-88fc-27cfe81c8262" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PerformanceStockUnitsGrantsInPeriodWeightedAverageGrantDateFairValue_aa67a73e-6461-420b-8924-4b2eaca322ca" xlink:href="ecvt-20231231.xsd#ecvt_PerformanceStockUnitsGrantsInPeriodWeightedAverageGrantDateFairValue"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_38a074a2-03d6-4cd9-88fc-27cfe81c8262" xlink:to="loc_ecvt_PerformanceStockUnitsGrantsInPeriodWeightedAverageGrantDateFairValue_aa67a73e-6461-420b-8924-4b2eaca322ca" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PerformanceStockUnitFairValueAssumptionsExpectedDividendRate_2f05c06d-cf4b-4268-8c2e-3490e68d6366" xlink:href="ecvt-20231231.xsd#ecvt_PerformanceStockUnitFairValueAssumptionsExpectedDividendRate"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_38a074a2-03d6-4cd9-88fc-27cfe81c8262" xlink:to="loc_ecvt_PerformanceStockUnitFairValueAssumptionsExpectedDividendRate_2f05c06d-cf4b-4268-8c2e-3490e68d6366" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PerformanceStockUnitFairValueAssumptionsRiskFreeInterestRate_730c773f-0b98-417d-9ea5-8a9bb62187b6" xlink:href="ecvt-20231231.xsd#ecvt_PerformanceStockUnitFairValueAssumptionsRiskFreeInterestRate"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_38a074a2-03d6-4cd9-88fc-27cfe81c8262" xlink:to="loc_ecvt_PerformanceStockUnitFairValueAssumptionsRiskFreeInterestRate_730c773f-0b98-417d-9ea5-8a9bb62187b6" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PerformanceStockUnitFairValueAssumptionsExpectedVolatilityRate_96b63a55-823d-4bc0-a894-7a0ff4fc8f1d" xlink:href="ecvt-20231231.xsd#ecvt_PerformanceStockUnitFairValueAssumptionsExpectedVolatilityRate"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_38a074a2-03d6-4cd9-88fc-27cfe81c8262" xlink:to="loc_ecvt_PerformanceStockUnitFairValueAssumptionsExpectedVolatilityRate_96b63a55-823d-4bc0-a894-7a0ff4fc8f1d" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PerformanceStockUnitFairValueAssumptionsExpectedTerm_7496ef13-8e51-415c-9c95-5add08b8ff73" xlink:href="ecvt-20231231.xsd#ecvt_PerformanceStockUnitFairValueAssumptionsExpectedTerm"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_38a074a2-03d6-4cd9-88fc-27cfe81c8262" xlink:to="loc_ecvt_PerformanceStockUnitFairValueAssumptionsExpectedTerm_7496ef13-8e51-415c-9c95-5add08b8ff73" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_91aa6273-0fbe-46e9-ac36-43d73be664e4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_eacd0824-48aa-4e75-8cf7-ab7faa6ea1ee" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_91aa6273-0fbe-46e9-ac36-43d73be664e4" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_97bce7be-096e-422c-acad-2fb974481f67" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_91aa6273-0fbe-46e9-ac36-43d73be664e4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_97bce7be-096e-422c-acad-2fb974481f67" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_5bc3bd28-3e1e-46dd-a438-f3c8eef8ebc5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_91aa6273-0fbe-46e9-ac36-43d73be664e4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_5bc3bd28-3e1e-46dd-a438-f3c8eef8ebc5" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_9b907d8b-162b-4d0b-b016-e2a22a4fc8a0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_91aa6273-0fbe-46e9-ac36-43d73be664e4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_9b907d8b-162b-4d0b-b016-e2a22a4fc8a0" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_6641de6c-df98-4ff7-87d2-e080dc1eebf4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_91aa6273-0fbe-46e9-ac36-43d73be664e4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_6641de6c-df98-4ff7-87d2-e080dc1eebf4" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_fab594c1-c14f-4867-aa43-8251e179ed19" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_bae829b4-5f04-479a-bd82-ae86a3f5fed0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_eacd0824-48aa-4e75-8cf7-ab7faa6ea1ee" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_bae829b4-5f04-479a-bd82-ae86a3f5fed0" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_1e14526f-9b82-4ecd-b700-83fdf39c10fb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_bae829b4-5f04-479a-bd82-ae86a3f5fed0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_1e14526f-9b82-4ecd-b700-83fdf39c10fb" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_1a98aed8-c0cb-4bce-9ccc-6c56e44c6b5a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_bae829b4-5f04-479a-bd82-ae86a3f5fed0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_1a98aed8-c0cb-4bce-9ccc-6c56e44c6b5a" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_6f060dcf-3081-4491-b7c5-aba7833bd086" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_bae829b4-5f04-479a-bd82-ae86a3f5fed0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_6f060dcf-3081-4491-b7c5-aba7833bd086" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_5d713ef7-17b7-4d91-8096-6d75d665742f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_bae829b4-5f04-479a-bd82-ae86a3f5fed0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_5d713ef7-17b7-4d91-8096-6d75d665742f" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_54ba9d64-e013-4b51-94f5-f2a1dcc1fabb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1_b2c113e1-b520-4266-9839-dc21efc331d8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_eacd0824-48aa-4e75-8cf7-ab7faa6ea1ee" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1_b2c113e1-b520-4266-9839-dc21efc331d8" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_ea2b828a-ee9c-4dd4-9b22-d002165d4262" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_eacd0824-48aa-4e75-8cf7-ab7faa6ea1ee" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_ea2b828a-ee9c-4dd4-9b22-d002165d4262" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_7d9b30c5-d2cc-44a8-8c4d-c982f408747a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_eacd0824-48aa-4e75-8cf7-ab7faa6ea1ee" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_7d9b30c5-d2cc-44a8-8c4d-c982f408747a" xlink:type="arc" order="5"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/StockBasedCompensationTotalStockBasedCompensationExpenseDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#StockBasedCompensationTotalStockBasedCompensationExpenseDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/StockBasedCompensationTotalStockBasedCompensationExpenseDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e1fdb3e4-ef09-4c21-8161-18c3422669c1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4e301014-d217-499a-9e6b-c65051cfdc94" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e1fdb3e4-ef09-4c21-8161-18c3422669c1" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4e301014-d217-499a-9e6b-c65051cfdc94" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_46787a73-1dd9-438e-bb47-4beb384602a6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AwardTypeAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4e301014-d217-499a-9e6b-c65051cfdc94" xlink:to="loc_us-gaap_AwardTypeAxis_46787a73-1dd9-438e-bb47-4beb384602a6" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_46787a73-1dd9-438e-bb47-4beb384602a6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_46787a73-1dd9-438e-bb47-4beb384602a6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_46787a73-1dd9-438e-bb47-4beb384602a6_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1a9d8f32-33d7-4240-b5f2-2fbe2d0cbb57" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_46787a73-1dd9-438e-bb47-4beb384602a6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1a9d8f32-33d7-4240-b5f2-2fbe2d0cbb57" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_64b44e6f-4faa-4a39-836a-246492fc9557" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EmployeeStockOptionMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1a9d8f32-33d7-4240-b5f2-2fbe2d0cbb57" xlink:to="loc_us-gaap_EmployeeStockOptionMember_64b44e6f-4faa-4a39-836a-246492fc9557" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_60f0a6d6-cddf-45cc-b007-afdf8b7cf8aa" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestrictedStockMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1a9d8f32-33d7-4240-b5f2-2fbe2d0cbb57" xlink:to="loc_us-gaap_RestrictedStockMember_60f0a6d6-cddf-45cc-b007-afdf8b7cf8aa" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_d200edab-ff02-4e90-83a4-e5a0a867d798" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1a9d8f32-33d7-4240-b5f2-2fbe2d0cbb57" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_d200edab-ff02-4e90-83a4-e5a0a867d798" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PerformanceStockUnitsMember_5740e592-fed9-45de-b0d4-6019878dc652" xlink:href="ecvt-20231231.xsd#ecvt_PerformanceStockUnitsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1a9d8f32-33d7-4240-b5f2-2fbe2d0cbb57" xlink:to="loc_ecvt_PerformanceStockUnitsMember_5740e592-fed9-45de-b0d4-6019878dc652" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_18ca118c-ceff-4b37-9db3-a255502f69a8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PerformanceSharesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1a9d8f32-33d7-4240-b5f2-2fbe2d0cbb57" xlink:to="loc_us-gaap_PerformanceSharesMember_18ca118c-ceff-4b37-9db3-a255502f69a8" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_3a5cbb34-c428-410c-bb35-1b9c1e0a0fb0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_VestingAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4e301014-d217-499a-9e6b-c65051cfdc94" xlink:to="loc_us-gaap_VestingAxis_3a5cbb34-c428-410c-bb35-1b9c1e0a0fb0" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_3a5cbb34-c428-410c-bb35-1b9c1e0a0fb0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_VestingDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_3a5cbb34-c428-410c-bb35-1b9c1e0a0fb0" xlink:to="loc_us-gaap_VestingDomain_3a5cbb34-c428-410c-bb35-1b9c1e0a0fb0_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_a4902c17-7bb6-487b-a548-96a93814ee60" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_VestingDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_3a5cbb34-c428-410c-bb35-1b9c1e0a0fb0" xlink:to="loc_us-gaap_VestingDomain_a4902c17-7bb6-487b-a548-96a93814ee60" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_SharebasedCompensationAwardTrancheFourMember_3e0b6db8-dd8a-435d-ad4c-d994c50dd315" xlink:href="ecvt-20231231.xsd#ecvt_SharebasedCompensationAwardTrancheFourMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_a4902c17-7bb6-487b-a548-96a93814ee60" xlink:to="loc_ecvt_SharebasedCompensationAwardTrancheFourMember_3e0b6db8-dd8a-435d-ad4c-d994c50dd315" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_6b41a34c-9f6b-403d-9696-0701fbbca282" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e1fdb3e4-ef09-4c21-8161-18c3422669c1" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_6b41a34c-9f6b-403d-9696-0701fbbca282" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense_daa9d257-754f-4699-bf81-17a24b28ec27" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e1fdb3e4-ef09-4c21-8161-18c3422669c1" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense_daa9d257-754f-4699-bf81-17a24b28ec27" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_f0555abb-932f-4dc6-b5d3-892cbcb9821b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e1fdb3e4-ef09-4c21-8161-18c3422669c1" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_f0555abb-932f-4dc6-b5d3-892cbcb9821b" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_42323589-2066-4e75-99e9-0f8396736b91" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e1fdb3e4-ef09-4c21-8161-18c3422669c1" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_42323589-2066-4e75-99e9-0f8396736b91" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_e78f9bc8-4ece-40e9-a8cc-08eba5b193ff" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e1fdb3e4-ef09-4c21-8161-18c3422669c1" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_e78f9bc8-4ece-40e9-a8cc-08eba5b193ff" xlink:type="arc" order="4"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/AntidilutiveSharesDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#AntidilutiveSharesDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/AntidilutiveSharesDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_84dbb70b-b7da-4dc8-aac2-f7bfdce50cab" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_2c26d4c8-de24-4dc7-aa2e-c673df8144dc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_84dbb70b-b7da-4dc8-aac2-f7bfdce50cab" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_2c26d4c8-de24-4dc7-aa2e-c673df8144dc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_977c484b-b300-4cf9-955d-a03652ac6ebf" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_2c26d4c8-de24-4dc7-aa2e-c673df8144dc" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_977c484b-b300-4cf9-955d-a03652ac6ebf" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_977c484b-b300-4cf9-955d-a03652ac6ebf_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_977c484b-b300-4cf9-955d-a03652ac6ebf" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_977c484b-b300-4cf9-955d-a03652ac6ebf_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_192ee75d-3f4b-4896-afa8-989375d0b12d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_977c484b-b300-4cf9-955d-a03652ac6ebf" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_192ee75d-3f4b-4896-afa8-989375d0b12d" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_57fd6112-b06e-4997-85d3-66131530364d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestrictedStockMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_192ee75d-3f4b-4896-afa8-989375d0b12d" xlink:to="loc_us-gaap_RestrictedStockMember_57fd6112-b06e-4997-85d3-66131530364d" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_94c52846-dff0-4dd9-8a2a-75933da9f15a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PerformanceSharesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_192ee75d-3f4b-4896-afa8-989375d0b12d" xlink:to="loc_us-gaap_PerformanceSharesMember_94c52846-dff0-4dd9-8a2a-75933da9f15a" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_RestrictedStockRestrictedStockUnitsRSUsandPerformanceStockUnitsPSUsMember_01828149-611f-4e0a-a2c9-f3a1e0cca021" xlink:href="ecvt-20231231.xsd#ecvt_RestrictedStockRestrictedStockUnitsRSUsandPerformanceStockUnitsPSUsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_192ee75d-3f4b-4896-afa8-989375d0b12d" xlink:to="loc_ecvt_RestrictedStockRestrictedStockUnitsRSUsandPerformanceStockUnitsPSUsMember_01828149-611f-4e0a-a2c9-f3a1e0cca021" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_9c8ae005-b959-4f47-a477-f8a372675b23" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EmployeeStockOptionMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_192ee75d-3f4b-4896-afa8-989375d0b12d" xlink:to="loc_us-gaap_EmployeeStockOptionMember_9c8ae005-b959-4f47-a477-f8a372675b23" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_2b1322e1-fcb6-4da0-a12a-f62d62b08d58" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_84dbb70b-b7da-4dc8-aac2-f7bfdce50cab" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_2b1322e1-fcb6-4da0-a12a-f62d62b08d58" xlink:type="arc" order="0"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/CommitmentsandContingentLiabilitiesNarrativesDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#CommitmentsandContingentLiabilitiesNarrativesDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/CommitmentsandContingentLiabilitiesNarrativesDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesLineItems_f7084041-cf94-4bf6-b014-c8bf95173d45" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LossContingenciesLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesTable_58fd8209-8717-428b-86f3-f377e61c1e9e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LossContingenciesTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_f7084041-cf94-4bf6-b014-c8bf95173d45" xlink:to="loc_us-gaap_LossContingenciesTable_58fd8209-8717-428b-86f3-f377e61c1e9e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_fe280d91-63f7-44b3-b85b-bf7441d78d08" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_58fd8209-8717-428b-86f3-f377e61c1e9e" xlink:to="loc_us-gaap_DebtInstrumentAxis_fe280d91-63f7-44b3-b85b-bf7441d78d08" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_fe280d91-63f7-44b3-b85b-bf7441d78d08_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_fe280d91-63f7-44b3-b85b-bf7441d78d08" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_fe280d91-63f7-44b3-b85b-bf7441d78d08_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_2b43b884-e8e7-4556-ac8e-5374a93d9bce" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_fe280d91-63f7-44b3-b85b-bf7441d78d08" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_2b43b884-e8e7-4556-ac8e-5374a93d9bce" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_37b76881-5c2a-42f3-b859-635e53f56cbc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevolvingCreditFacilityMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_2b43b884-e8e7-4556-ac8e-5374a93d9bce" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_37b76881-5c2a-42f3-b859-635e53f56cbc" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EnvironmentalRemediationContingencyAxis_ce5e6db0-05f7-4615-b9d0-ec775bf8ff65" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EnvironmentalRemediationContingencyAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_58fd8209-8717-428b-86f3-f377e61c1e9e" xlink:to="loc_us-gaap_EnvironmentalRemediationContingencyAxis_ce5e6db0-05f7-4615-b9d0-ec775bf8ff65" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EnvironmentalRemediationContingencyDomain_ce5e6db0-05f7-4615-b9d0-ec775bf8ff65_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EnvironmentalRemediationContingencyDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EnvironmentalRemediationContingencyAxis_ce5e6db0-05f7-4615-b9d0-ec775bf8ff65" xlink:to="loc_us-gaap_EnvironmentalRemediationContingencyDomain_ce5e6db0-05f7-4615-b9d0-ec775bf8ff65_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EnvironmentalRemediationContingencyDomain_8a04019f-ed9b-44ee-b670-c7fb5146f073" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EnvironmentalRemediationContingencyDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EnvironmentalRemediationContingencyAxis_ce5e6db0-05f7-4615-b9d0-ec775bf8ff65" xlink:to="loc_us-gaap_EnvironmentalRemediationContingencyDomain_8a04019f-ed9b-44ee-b670-c7fb5146f073" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_SubsurfaceRemediationAndWetlandsManagementMember_083a48a9-84da-40c1-a4db-0cec0ec889f3" xlink:href="ecvt-20231231.xsd#ecvt_SubsurfaceRemediationAndWetlandsManagementMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EnvironmentalRemediationContingencyDomain_8a04019f-ed9b-44ee-b670-c7fb5146f073" xlink:to="loc_ecvt_SubsurfaceRemediationAndWetlandsManagementMember_083a48a9-84da-40c1-a4db-0cec0ec889f3" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_SubsurfaceRemediationAndSoilVaporExtractionMember_a3d9fcf4-6185-4681-8269-1c304bd65fa9" xlink:href="ecvt-20231231.xsd#ecvt_SubsurfaceRemediationAndSoilVaporExtractionMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EnvironmentalRemediationContingencyDomain_8a04019f-ed9b-44ee-b670-c7fb5146f073" xlink:to="loc_ecvt_SubsurfaceRemediationAndSoilVaporExtractionMember_a3d9fcf4-6185-4681-8269-1c304bd65fa9" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccrualForEnvironmentalLossContingencies_232ea36b-feb7-4201-960c-e460408e2068" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccrualForEnvironmentalLossContingencies"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_f7084041-cf94-4bf6-b014-c8bf95173d45" xlink:to="loc_us-gaap_AccrualForEnvironmentalLossContingencies_232ea36b-feb7-4201-960c-e460408e2068" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_37df7a99-c6b2-4ec4-a586-1f010fdb8dc9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LettersOfCreditOutstandingAmount"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_f7084041-cf94-4bf6-b014-c8bf95173d45" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_37df7a99-c6b2-4ec4-a586-1f010fdb8dc9" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_EnvironmentalLossContingencyStatementOfFinancialPositionExtensibleEnumerationNotDisclosedFlag_cd707666-b1dc-4450-8947-4ab98f50c42b" xlink:href="ecvt-20231231.xsd#ecvt_EnvironmentalLossContingencyStatementOfFinancialPositionExtensibleEnumerationNotDisclosedFlag"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_f7084041-cf94-4bf6-b014-c8bf95173d45" xlink:to="loc_ecvt_EnvironmentalLossContingencyStatementOfFinancialPositionExtensibleEnumerationNotDisclosedFlag_cd707666-b1dc-4450-8947-4ab98f50c42b" xlink:type="arc" order="2"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/RelatedPartyTransactionsDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#RelatedPartyTransactionsDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/RelatedPartyTransactionsDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_2b3b0a7b-8c31-47b2-b97e-1aac1002e208" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RelatedPartyTransactionLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_c1d96d1e-d707-4a9e-bd80-02a590db9536" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_2b3b0a7b-8c31-47b2-b97e-1aac1002e208" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_c1d96d1e-d707-4a9e-bd80-02a590db9536" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_cd32d793-fae1-4203-99d9-78fae830f2f5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_c1d96d1e-d707-4a9e-bd80-02a590db9536" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_cd32d793-fae1-4203-99d9-78fae830f2f5" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_cd32d793-fae1-4203-99d9-78fae830f2f5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RelatedPartyDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_cd32d793-fae1-4203-99d9-78fae830f2f5" xlink:to="loc_us-gaap_RelatedPartyDomain_cd32d793-fae1-4203-99d9-78fae830f2f5_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_6faf87da-0d2f-4a0e-a077-b3db751348a6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RelatedPartyDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_cd32d793-fae1-4203-99d9-78fae830f2f5" xlink:to="loc_us-gaap_RelatedPartyDomain_6faf87da-0d2f-4a0e-a077-b3db751348a6" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_65d507d9-43cc-4ad9-a693-e0e331e7b067" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RelatedPartyMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_6faf87da-0d2f-4a0e-a077-b3db751348a6" xlink:to="loc_us-gaap_RelatedPartyMember_65d507d9-43cc-4ad9-a693-e0e331e7b067" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionAxis_c9c6cc36-6baf-41ae-ae37-6ea5f1a807b5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RelatedPartyTransactionAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_c1d96d1e-d707-4a9e-bd80-02a590db9536" xlink:to="loc_us-gaap_RelatedPartyTransactionAxis_c9c6cc36-6baf-41ae-ae37-6ea5f1a807b5" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_c9c6cc36-6baf-41ae-ae37-6ea5f1a807b5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RelatedPartyTransactionDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_c9c6cc36-6baf-41ae-ae37-6ea5f1a807b5" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_c9c6cc36-6baf-41ae-ae37-6ea5f1a807b5_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_4692e862-0910-4bff-809f-1aa3ec792114" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RelatedPartyTransactionDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_c9c6cc36-6baf-41ae-ae37-6ea5f1a807b5" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_4692e862-0910-4bff-809f-1aa3ec792114" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_OperatingLeaseRentalPaymentsMember_9e18da3d-3731-4cee-97a3-012319520cc6" xlink:href="ecvt-20231231.xsd#ecvt_OperatingLeaseRentalPaymentsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_4692e862-0910-4bff-809f-1aa3ec792114" xlink:to="loc_ecvt_OperatingLeaseRentalPaymentsMember_9e18da3d-3731-4cee-97a3-012319520cc6" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_SalesfromPartnershiptoCompanyMember_2c6a5c43-c86d-4fc5-a979-5936c3fcb191" xlink:href="ecvt-20231231.xsd#ecvt_SalesfromPartnershiptoCompanyMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_4692e862-0910-4bff-809f-1aa3ec792114" xlink:to="loc_ecvt_SalesfromPartnershiptoCompanyMember_2c6a5c43-c86d-4fc5-a979-5936c3fcb191" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_ManufacturingCostsMember_e5507c87-9d57-4bfd-873d-da4f46834a87" xlink:href="ecvt-20231231.xsd#ecvt_ManufacturingCostsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_4692e862-0910-4bff-809f-1aa3ec792114" xlink:to="loc_ecvt_ManufacturingCostsMember_e5507c87-9d57-4bfd-873d-da4f46834a87" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_ServicesMember_d969b718-31fc-4588-8a90-d18ce8aa4559" xlink:href="ecvt-20231231.xsd#ecvt_ServicesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_4692e862-0910-4bff-809f-1aa3ec792114" xlink:to="loc_ecvt_ServicesMember_d969b718-31fc-4588-8a90-d18ce8aa4559" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_ProductDemonstrationCostsMember_71af270b-8793-40f9-a58e-1af58f4d4036" xlink:href="ecvt-20231231.xsd#ecvt_ProductDemonstrationCostsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_4692e862-0910-4bff-809f-1aa3ec792114" xlink:to="loc_ecvt_ProductDemonstrationCostsMember_71af270b-8793-40f9-a58e-1af58f4d4036" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_772a2116-1cac-418c-a659-3e635f6b436f" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_CounterpartyNameAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_c1d96d1e-d707-4a9e-bd80-02a590db9536" xlink:to="loc_srt_CounterpartyNameAxis_772a2116-1cac-418c-a659-3e635f6b436f" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_772a2116-1cac-418c-a659-3e635f6b436f_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_772a2116-1cac-418c-a659-3e635f6b436f" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_772a2116-1cac-418c-a659-3e635f6b436f_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_7a7b1d91-0060-48ae-aa8f-43ac08e46b36" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_772a2116-1cac-418c-a659-3e635f6b436f" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_7a7b1d91-0060-48ae-aa8f-43ac08e46b36" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateJointVentureMember_5ec03a7e-72fc-4f97-ad12-dbd69ad596ed" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CorporateJointVentureMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_7a7b1d91-0060-48ae-aa8f-43ac08e46b36" xlink:to="loc_us-gaap_CorporateJointVentureMember_5ec03a7e-72fc-4f97-ad12-dbd69ad596ed" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_INEOSCapitalPartnersMember_f32c067f-a513-432e-9a91-55e0999fa2b2" xlink:href="ecvt-20231231.xsd#ecvt_INEOSCapitalPartnersMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_7a7b1d91-0060-48ae-aa8f-43ac08e46b36" xlink:to="loc_ecvt_INEOSCapitalPartnersMember_f32c067f-a513-432e-9a91-55e0999fa2b2" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_5448a499-093a-421d-a1f2-4187466d8066" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_Revenues"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_2b3b0a7b-8c31-47b2-b97e-1aac1002e208" xlink:to="loc_us-gaap_Revenues_5448a499-093a-421d-a1f2-4187466d8066" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionPurchasesFromRelatedParty_87f738e9-8071-4526-9c32-b97c068e5753" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RelatedPartyTransactionPurchasesFromRelatedParty"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_2b3b0a7b-8c31-47b2-b97e-1aac1002e208" xlink:to="loc_us-gaap_RelatedPartyTransactionPurchasesFromRelatedParty_87f738e9-8071-4526-9c32-b97c068e5753" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNet_4d8cb07b-651c-4acb-991d-1d1e56834266" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccountsReceivableNet"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_2b3b0a7b-8c31-47b2-b97e-1aac1002e208" xlink:to="loc_us-gaap_AccountsReceivableNet_4d8cb07b-651c-4acb-991d-1d1e56834266" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrent_32990396-8d95-44fa-b364-c4a3dacc723d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccountsPayableCurrent"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_2b3b0a7b-8c31-47b2-b97e-1aac1002e208" xlink:to="loc_us-gaap_AccountsPayableCurrent_32990396-8d95-44fa-b364-c4a3dacc723d" xlink:type="arc" order="3"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/ScheduleICondensedStatementsofIncomeDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#ScheduleICondensedStatementsofIncomeDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/ScheduleICondensedStatementsofIncomeDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_srt_CondensedFinancialStatementsCaptionsLineItems_fb586dc0-d62b-4e80-82c3-a2247fcb4608" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_CondensedFinancialStatementsCaptionsLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfCondensedFinancialStatementsTable_a2912708-229b-4b9a-8843-1fdf31a1b837" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ScheduleOfCondensedFinancialStatementsTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_fb586dc0-d62b-4e80-82c3-a2247fcb4608" xlink:to="loc_srt_ScheduleOfCondensedFinancialStatementsTable_a2912708-229b-4b9a-8843-1fdf31a1b837" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_5388129b-f8ce-4197-aaa5-600f1de18720" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidatedEntitiesAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_srt_ScheduleOfCondensedFinancialStatementsTable_a2912708-229b-4b9a-8843-1fdf31a1b837" xlink:to="loc_srt_ConsolidatedEntitiesAxis_5388129b-f8ce-4197-aaa5-600f1de18720" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_5388129b-f8ce-4197-aaa5-600f1de18720_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidatedEntitiesDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_5388129b-f8ce-4197-aaa5-600f1de18720" xlink:to="loc_srt_ConsolidatedEntitiesDomain_5388129b-f8ce-4197-aaa5-600f1de18720_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_545e8ebb-7e28-4480-9bf2-5470e11b82ad" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidatedEntitiesDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_5388129b-f8ce-4197-aaa5-600f1de18720" xlink:to="loc_srt_ConsolidatedEntitiesDomain_545e8ebb-7e28-4480-9bf2-5470e11b82ad" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ParentCompanyMember_b1857ca3-6778-44db-806a-09495ee8a57e" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ParentCompanyMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_545e8ebb-7e28-4480-9bf2-5470e11b82ad" xlink:to="loc_srt_ParentCompanyMember_b1857ca3-6778-44db-806a-09495ee8a57e" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_b1980f56-8443-46a5-93ce-73074c277bc8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensation"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_fb586dc0-d62b-4e80-82c3-a2247fcb4608" xlink:to="loc_us-gaap_ShareBasedCompensation_b1980f56-8443-46a5-93ce-73074c277bc8" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromSubsidiariesNetOfTax_c60feef0-64ce-4ca9-89e9-462f751ebca7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeLossFromSubsidiariesNetOfTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_fb586dc0-d62b-4e80-82c3-a2247fcb4608" xlink:to="loc_us-gaap_IncomeLossFromSubsidiariesNetOfTax_c60feef0-64ce-4ca9-89e9-462f751ebca7" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_772cfb98-3871-46dd-99bb-03ce2593680f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NetIncomeLoss"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_fb586dc0-d62b-4e80-82c3-a2247fcb4608" xlink:to="loc_us-gaap_NetIncomeLoss_772cfb98-3871-46dd-99bb-03ce2593680f" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract_41cddf81-38bb-42cb-b884-9a2bcfa32443" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_fb586dc0-d62b-4e80-82c3-a2247fcb4608" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract_41cddf81-38bb-42cb-b884-9a2bcfa32443" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPortionAttributableToParent_38280285-90c3-4b32-8711-f81e47c07232" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPortionAttributableToParent"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract_41cddf81-38bb-42cb-b884-9a2bcfa32443" xlink:to="loc_us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPortionAttributableToParent_38280285-90c3-4b32-8711-f81e47c07232" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationTaxParent_71307336-94d4-4a2e-bdf9-aeaac2a81b81" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationTaxParent"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract_41cddf81-38bb-42cb-b884-9a2bcfa32443" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationTaxParent_71307336-94d4-4a2e-bdf9-aeaac2a81b81" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent_db965675-c32d-4390-80a3-f575f687886c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract_41cddf81-38bb-42cb-b884-9a2bcfa32443" xlink:to="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent_db965675-c32d-4390-80a3-f575f687886c" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_df4272a5-2566-47a5-9fdd-b522b729f239" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract_41cddf81-38bb-42cb-b884-9a2bcfa32443" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_df4272a5-2566-47a5-9fdd-b522b729f239" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComprehensiveIncomeNetOfTax_a545a4b7-1c30-49d0-b0e7-2d828272f150" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ComprehensiveIncomeNetOfTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_fb586dc0-d62b-4e80-82c3-a2247fcb4608" xlink:to="loc_us-gaap_ComprehensiveIncomeNetOfTax_a545a4b7-1c30-49d0-b0e7-2d828272f150" xlink:type="arc" order="4"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/ScheduleICondensedBalanceSheetDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#ScheduleICondensedBalanceSheetDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/ScheduleICondensedBalanceSheetDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_srt_CondensedFinancialStatementsCaptionsLineItems_73442976-9580-494e-9419-85808bdc0b97" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_CondensedFinancialStatementsCaptionsLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfCondensedFinancialStatementsTable_ec011a04-269f-47b0-ae91-c3480d43a0d9" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ScheduleOfCondensedFinancialStatementsTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_73442976-9580-494e-9419-85808bdc0b97" xlink:to="loc_srt_ScheduleOfCondensedFinancialStatementsTable_ec011a04-269f-47b0-ae91-c3480d43a0d9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_408edb0e-774c-4f81-abda-7411ea033ba1" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidatedEntitiesAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_srt_ScheduleOfCondensedFinancialStatementsTable_ec011a04-269f-47b0-ae91-c3480d43a0d9" xlink:to="loc_srt_ConsolidatedEntitiesAxis_408edb0e-774c-4f81-abda-7411ea033ba1" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_408edb0e-774c-4f81-abda-7411ea033ba1_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidatedEntitiesDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_408edb0e-774c-4f81-abda-7411ea033ba1" xlink:to="loc_srt_ConsolidatedEntitiesDomain_408edb0e-774c-4f81-abda-7411ea033ba1_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_7d4528b0-72f3-440b-8b97-cbaa040b3411" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidatedEntitiesDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_408edb0e-774c-4f81-abda-7411ea033ba1" xlink:to="loc_srt_ConsolidatedEntitiesDomain_7d4528b0-72f3-440b-8b97-cbaa040b3411" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ParentCompanyMember_ad02d9ab-b8a3-454e-8ba3-81538df4ac37" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ParentCompanyMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_7d4528b0-72f3-440b-8b97-cbaa040b3411" xlink:to="loc_srt_ParentCompanyMember_ad02d9ab-b8a3-454e-8ba3-81538df4ac37" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_bc652b1b-02b0-4b3d-8d7e-75b39171395b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AssetsAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_73442976-9580-494e-9419-85808bdc0b97" xlink:to="loc_us-gaap_AssetsAbstract_bc652b1b-02b0-4b3d-8d7e-75b39171395b" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures_b2937e7c-30cb-4e18-b459-41a66ea91437" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_bc652b1b-02b0-4b3d-8d7e-75b39171395b" xlink:to="loc_us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures_b2937e7c-30cb-4e18-b459-41a66ea91437" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_3d2d25e2-761c-404a-8664-0eb21c467030" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_Assets"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_bc652b1b-02b0-4b3d-8d7e-75b39171395b" xlink:to="loc_us-gaap_Assets_3d2d25e2-761c-404a-8664-0eb21c467030" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAbstract_3dd46f0f-1d50-41d4-8d50-b1c0ce9213af" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LiabilitiesAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_73442976-9580-494e-9419-85808bdc0b97" xlink:to="loc_us-gaap_LiabilitiesAbstract_3dd46f0f-1d50-41d4-8d50-b1c0ce9213af" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_Liabilities_db194556-2770-4b37-a921-2d8accea04ab" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_Liabilities"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_3dd46f0f-1d50-41d4-8d50-b1c0ce9213af" xlink:to="loc_us-gaap_Liabilities_db194556-2770-4b37-a921-2d8accea04ab" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_a05d7feb-828a-43db-ad27-2b8c8c9d8876" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_73442976-9580-494e-9419-85808bdc0b97" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_a05d7feb-828a-43db-ad27-2b8c8c9d8876" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockValue_6129e831-75f4-41b2-8c75-6dffa324e91a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockValue"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_a05d7feb-828a-43db-ad27-2b8c8c9d8876" xlink:to="loc_us-gaap_CommonStockValue_6129e831-75f4-41b2-8c75-6dffa324e91a" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockValue_7220f36e-89f8-4085-ac22-17d5852fe2c8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PreferredStockValue"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_a05d7feb-828a-43db-ad27-2b8c8c9d8876" xlink:to="loc_us-gaap_PreferredStockValue_7220f36e-89f8-4085-ac22-17d5852fe2c8" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapital_338e1aa6-4a7b-463a-be72-654a7bb9df51" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AdditionalPaidInCapital"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_a05d7feb-828a-43db-ad27-2b8c8c9d8876" xlink:to="loc_us-gaap_AdditionalPaidInCapital_338e1aa6-4a7b-463a-be72-654a7bb9df51" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_69a760a3-8b35-40f1-80fe-2cf69e0d534c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_a05d7feb-828a-43db-ad27-2b8c8c9d8876" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_69a760a3-8b35-40f1-80fe-2cf69e0d534c" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockValue_3c8ad5b4-2186-4a1e-9256-bbe3a3ef7029" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TreasuryStockValue"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_a05d7feb-828a-43db-ad27-2b8c8c9d8876" xlink:to="loc_us-gaap_TreasuryStockValue_3c8ad5b4-2186-4a1e-9256-bbe3a3ef7029" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_f335d026-794e-41ea-92ec-b8fece7fc713" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_a05d7feb-828a-43db-ad27-2b8c8c9d8876" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_f335d026-794e-41ea-92ec-b8fece7fc713" xlink:type="arc" order="6"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_e4f2382b-3c25-4e84-a1a4-e1b9c1b30be9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockholdersEquity"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_a05d7feb-828a-43db-ad27-2b8c8c9d8876" xlink:to="loc_us-gaap_StockholdersEquity_e4f2382b-3c25-4e84-a1a4-e1b9c1b30be9" xlink:type="arc" order="7"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquity_b2269ac7-3fc0-4735-8c46-0f1c2c0400bc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LiabilitiesAndStockholdersEquity"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_a05d7feb-828a-43db-ad27-2b8c8c9d8876" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquity_b2269ac7-3fc0-4735-8c46-0f1c2c0400bc" xlink:type="arc" order="8"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/ScheduleICondensedBalanceSheetSharesDataDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#ScheduleICondensedBalanceSheetSharesDataDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/ScheduleICondensedBalanceSheetSharesDataDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_srt_CondensedFinancialStatementsCaptionsLineItems_3cfbf958-4a85-4f02-b266-1f199bda8545" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_CondensedFinancialStatementsCaptionsLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfCondensedFinancialStatementsTable_acccdd96-6a6c-4969-b044-074a53585437" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ScheduleOfCondensedFinancialStatementsTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_3cfbf958-4a85-4f02-b266-1f199bda8545" xlink:to="loc_srt_ScheduleOfCondensedFinancialStatementsTable_acccdd96-6a6c-4969-b044-074a53585437" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_7fca95d5-2f3a-482f-b851-0db1c4f22130" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidatedEntitiesAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_srt_ScheduleOfCondensedFinancialStatementsTable_acccdd96-6a6c-4969-b044-074a53585437" xlink:to="loc_srt_ConsolidatedEntitiesAxis_7fca95d5-2f3a-482f-b851-0db1c4f22130" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_7fca95d5-2f3a-482f-b851-0db1c4f22130_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidatedEntitiesDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_7fca95d5-2f3a-482f-b851-0db1c4f22130" xlink:to="loc_srt_ConsolidatedEntitiesDomain_7fca95d5-2f3a-482f-b851-0db1c4f22130_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_ef1fb491-1b8e-4a82-a679-ff6b2b7d38de" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidatedEntitiesDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_7fca95d5-2f3a-482f-b851-0db1c4f22130" xlink:to="loc_srt_ConsolidatedEntitiesDomain_ef1fb491-1b8e-4a82-a679-ff6b2b7d38de" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ParentCompanyMember_fbf24e29-a985-44c4-95bf-306f2423704d" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ParentCompanyMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_ef1fb491-1b8e-4a82-a679-ff6b2b7d38de" xlink:to="loc_srt_ParentCompanyMember_fbf24e29-a985-44c4-95bf-306f2423704d" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityAbstract_cacc950d-1556-434c-b958-3c891c0619f7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockholdersEquityAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_3cfbf958-4a85-4f02-b266-1f199bda8545" xlink:to="loc_us-gaap_StockholdersEquityAbstract_cacc950d-1556-434c-b958-3c891c0619f7" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_15372dd3-e22e-4bf9-8f7c-859177af9421" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_3cfbf958-4a85-4f02-b266-1f199bda8545" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_15372dd3-e22e-4bf9-8f7c-859177af9421" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_fb73c880-12ca-41d4-92d1-67b0319dd346" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockSharesAuthorized"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_3cfbf958-4a85-4f02-b266-1f199bda8545" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_fb73c880-12ca-41d4-92d1-67b0319dd346" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_1a7bb807-c907-4313-a52c-d15c2814b0ce" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockSharesIssued"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_3cfbf958-4a85-4f02-b266-1f199bda8545" xlink:to="loc_us-gaap_CommonStockSharesIssued_1a7bb807-c907-4313-a52c-d15c2814b0ce" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_5bd81017-d540-495e-a93d-eec4ab6beb83" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockSharesOutstanding"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_3cfbf958-4a85-4f02-b266-1f199bda8545" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_5bd81017-d540-495e-a93d-eec4ab6beb83" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockParOrStatedValuePerShare_fa2eb37b-76c1-46b7-a06e-366fe67b2ec2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PreferredStockParOrStatedValuePerShare"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_3cfbf958-4a85-4f02-b266-1f199bda8545" xlink:to="loc_us-gaap_PreferredStockParOrStatedValuePerShare_fa2eb37b-76c1-46b7-a06e-366fe67b2ec2" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_26993b3c-00c3-467c-89e9-2198d55a57ec" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PreferredStockSharesAuthorized"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_3cfbf958-4a85-4f02-b266-1f199bda8545" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_26993b3c-00c3-467c-89e9-2198d55a57ec" xlink:type="arc" order="6"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesIssued_ef581152-3537-46dc-9d0f-ac832a2436c9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PreferredStockSharesIssued"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_3cfbf958-4a85-4f02-b266-1f199bda8545" xlink:to="loc_us-gaap_PreferredStockSharesIssued_ef581152-3537-46dc-9d0f-ac832a2436c9" xlink:type="arc" order="7"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_19299588-25f8-4c67-8fc2-9e0639c86ec0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PreferredStockSharesOutstanding"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_3cfbf958-4a85-4f02-b266-1f199bda8545" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_19299588-25f8-4c67-8fc2-9e0639c86ec0" xlink:type="arc" order="8"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonShares_1dbc4bfc-624d-4a5a-bc0c-49eefa646b9e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TreasuryStockCommonShares"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_3cfbf958-4a85-4f02-b266-1f199bda8545" xlink:to="loc_us-gaap_TreasuryStockCommonShares_1dbc4bfc-624d-4a5a-bc0c-49eefa646b9e" xlink:type="arc" order="9"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/ScheduleICondensedStatementofCashFlowDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#ScheduleICondensedStatementofCashFlowDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/ScheduleICondensedStatementofCashFlowDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_srt_CondensedFinancialStatementsCaptionsLineItems_0950a511-8c19-4e8d-bece-68bdbfef0630" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_CondensedFinancialStatementsCaptionsLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfCondensedFinancialStatementsTable_5f01931d-f857-4176-9232-cdcfed2f63fa" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ScheduleOfCondensedFinancialStatementsTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_0950a511-8c19-4e8d-bece-68bdbfef0630" xlink:to="loc_srt_ScheduleOfCondensedFinancialStatementsTable_5f01931d-f857-4176-9232-cdcfed2f63fa" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_a7e7bafe-c89d-4e5f-920e-f64674722fa9" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidatedEntitiesAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_srt_ScheduleOfCondensedFinancialStatementsTable_5f01931d-f857-4176-9232-cdcfed2f63fa" xlink:to="loc_srt_ConsolidatedEntitiesAxis_a7e7bafe-c89d-4e5f-920e-f64674722fa9" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_a7e7bafe-c89d-4e5f-920e-f64674722fa9_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidatedEntitiesDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_a7e7bafe-c89d-4e5f-920e-f64674722fa9" xlink:to="loc_srt_ConsolidatedEntitiesDomain_a7e7bafe-c89d-4e5f-920e-f64674722fa9_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_8991b741-e40b-4451-b12b-52ce5a003e73" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidatedEntitiesDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_a7e7bafe-c89d-4e5f-920e-f64674722fa9" xlink:to="loc_srt_ConsolidatedEntitiesDomain_8991b741-e40b-4451-b12b-52ce5a003e73" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ParentCompanyMember_327d3e98-5747-49c1-96d4-2b1d3ceae8cc" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ParentCompanyMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_8991b741-e40b-4451-b12b-52ce5a003e73" xlink:to="loc_srt_ParentCompanyMember_327d3e98-5747-49c1-96d4-2b1d3ceae8cc" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_1ef18ce2-41ba-41ad-8598-f3b43e69c767" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_0950a511-8c19-4e8d-bece-68bdbfef0630" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_1ef18ce2-41ba-41ad-8598-f3b43e69c767" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_d3980967-cb38-4989-8b76-2c7afbd12b53" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NetIncomeLoss"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_1ef18ce2-41ba-41ad-8598-f3b43e69c767" xlink:to="loc_us-gaap_NetIncomeLoss_d3980967-cb38-4989-8b76-2c7afbd12b53" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromSubsidiariesNetOfTax_d12daf1c-eba8-4bb2-8050-b0490e06090e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeLossFromSubsidiariesNetOfTax"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_1ef18ce2-41ba-41ad-8598-f3b43e69c767" xlink:to="loc_us-gaap_IncomeLossFromSubsidiariesNetOfTax_d12daf1c-eba8-4bb2-8050-b0490e06090e" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_6158ca8f-f97a-4def-8b0d-c537778012a1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_1ef18ce2-41ba-41ad-8598-f3b43e69c767" xlink:to="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_6158ca8f-f97a-4def-8b0d-c537778012a1" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_c909a385-36f1-4239-8c7d-44425b6b2694" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensation"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_6158ca8f-f97a-4def-8b0d-c537778012a1" xlink:to="loc_us-gaap_ShareBasedCompensation_c909a385-36f1-4239-8c7d-44425b6b2694" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_9bbb7db7-5c89-4073-8085-7f5bff4092b3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_1ef18ce2-41ba-41ad-8598-f3b43e69c767" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_9bbb7db7-5c89-4073-8085-7f5bff4092b3" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentDividendsOrDistributions_ecbb84e8-e509-4d5d-aa55-5027cdd0da8c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityMethodInvestmentDividendsOrDistributions"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_0950a511-8c19-4e8d-bece-68bdbfef0630" xlink:to="loc_us-gaap_EquityMethodInvestmentDividendsOrDistributions_ecbb84e8-e509-4d5d-aa55-5027cdd0da8c" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_9f6612b2-9326-4de7-ab91-8050266009ab" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_0950a511-8c19-4e8d-bece-68bdbfef0630" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_9f6612b2-9326-4de7-ab91-8050266009ab" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_a3c3bd65-319f-4147-bd6a-130de978ad52" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_9f6612b2-9326-4de7-ab91-8050266009ab" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_a3c3bd65-319f-4147-bd6a-130de978ad52" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_cc77d447-e74e-460a-bc12-d21c8e0781b0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_0950a511-8c19-4e8d-bece-68bdbfef0630" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_cc77d447-e74e-460a-bc12-d21c8e0781b0" xlink:type="arc" order="3"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfOrdinaryDividends_cbbc15cc-076e-4177-b298-80bd575691ff" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PaymentsOfOrdinaryDividends"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_cc77d447-e74e-460a-bc12-d21c8e0781b0" xlink:to="loc_us-gaap_PaymentsOfOrdinaryDividends_cbbc15cc-076e-4177-b298-80bd575691ff" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_0cb397f6-8560-4284-9808-b9f4a6bf2340" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_cc77d447-e74e-460a-bc12-d21c8e0781b0" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_0cb397f6-8560-4284-9808-b9f4a6bf2340" xlink:type="arc" order="2"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_64f720a5-a1e6-4b59-b016-8b140c0c7601" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_0950a511-8c19-4e8d-bece-68bdbfef0630" xlink:to="loc_us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_64f720a5-a1e6-4b59-b016-8b140c0c7601" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_4a26520c-1fdb-41fc-8ba1-c9b22c854b2a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_0950a511-8c19-4e8d-bece-68bdbfef0630" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_4a26520c-1fdb-41fc-8ba1-c9b22c854b2a" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_16406f79-5dfc-4adf-a4af-5810d7431c83" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_0950a511-8c19-4e8d-bece-68bdbfef0630" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_16406f79-5dfc-4adf-a4af-5810d7431c83" xlink:type="arc" order="6"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_c987c0e2-1a46-4ded-afb6-749b001dd973" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>

</link:definitionLink>

<link:roleRef roleURI="http://www.ecovyst.com/role/ScheduleINarrativeDetails" xlink:type="simple" xlink:href="ecvt-20231231.xsd#ScheduleINarrativeDetails"/>

<link:definitionLink xlink:role="http://www.ecovyst.com/role/ScheduleINarrativeDetails" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_srt_CondensedFinancialStatementsCaptionsLineItems_28afaf3d-1de9-453c-af93-e77f86634577" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_CondensedFinancialStatementsCaptionsLineItems"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfCondensedFinancialStatementsTable_d86bf6af-df5e-4ac2-991a-f159b42b8a5e" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ScheduleOfCondensedFinancialStatementsTable"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_28afaf3d-1de9-453c-af93-e77f86634577" xlink:to="loc_srt_ScheduleOfCondensedFinancialStatementsTable_d86bf6af-df5e-4ac2-991a-f159b42b8a5e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_34cfd5e8-58ee-48eb-9a8d-9abc6c9a1c5a" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidatedEntitiesAxis"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_srt_ScheduleOfCondensedFinancialStatementsTable_d86bf6af-df5e-4ac2-991a-f159b42b8a5e" xlink:to="loc_srt_ConsolidatedEntitiesAxis_34cfd5e8-58ee-48eb-9a8d-9abc6c9a1c5a" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_34cfd5e8-58ee-48eb-9a8d-9abc6c9a1c5a_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidatedEntitiesDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_34cfd5e8-58ee-48eb-9a8d-9abc6c9a1c5a" xlink:to="loc_srt_ConsolidatedEntitiesDomain_34cfd5e8-58ee-48eb-9a8d-9abc6c9a1c5a_default" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_39aacd73-0cb2-4100-9653-bbfc43ded17f" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidatedEntitiesDomain"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_34cfd5e8-58ee-48eb-9a8d-9abc6c9a1c5a" xlink:to="loc_srt_ConsolidatedEntitiesDomain_39aacd73-0cb2-4100-9653-bbfc43ded17f" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_srt_ParentCompanyMember_865183ad-24d2-4b0c-a991-4ea39ce20a23" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ParentCompanyMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_39aacd73-0cb2-4100-9653-bbfc43ded17f" xlink:to="loc_srt_ParentCompanyMember_865183ad-24d2-4b0c-a991-4ea39ce20a23" xlink:type="arc" order="0"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockDividendsPerShareCashPaid_ed6c013a-f508-4fb0-b077-db766fae0c2d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockDividendsPerShareCashPaid"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CondensedFinancialStatementsCaptionsLineItems_28afaf3d-1de9-453c-af93-e77f86634577" xlink:to="loc_us-gaap_CommonStockDividendsPerShareCashPaid_ed6c013a-f508-4fb0-b077-db766fae0c2d" xlink:type="arc" order="0"/>

</link:definitionLink>

<link:roleRef roleURI="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" xlink:type="simple" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-roles-2023.xsd#eedm"/>

<link:definitionLink xlink:role="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AssetsAbstract"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet" xlink:type="arc" order="1"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LiabilitiesCurrentAbstract"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedLiabilitiesCurrent" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccruedLiabilitiesCurrent"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract" xlink:to="loc_us-gaap_AccruedLiabilitiesCurrent" xlink:type="arc" order="1"/>

</link:definitionLink>

<link:roleRef roleURI="http://xbrl.sec.gov/ecd/AwardTypeOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2023/ecd-2023.xsd#AwardTypeOnly"/>

<link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/AwardTypeOnly" xlink:type="extended">

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember" xlink:type="arc" order="4"/>

<link:loc xlink:type="locator" xlink:label="loc_ecvt_PerformanceStockUnitsMember_e56d2bb0-21c5-4ec3-888e-cf015dd9a5e0" xlink:href="ecvt-20231231.xsd#ecvt_PerformanceStockUnitsMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_ecvt_PerformanceStockUnitsMember_e56d2bb0-21c5-4ec3-888e-cf015dd9a5e0" xlink:type="arc" order="5"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestrictedStockMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_us-gaap_RestrictedStockMember" xlink:type="arc" order="6"/>

<link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PerformanceSharesMember"/>

<link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_us-gaap_PerformanceSharesMember" xlink:type="arc" order="7"/>

</link:definitionLink>

</link:linkbase>

18 Previous Filings that this Filing References

  As Of               Filer                 Filing    For·On·As Docs:Size             Issuer                      Filing Agent

 2/28/23  Ecovyst Inc.                      10-K       12/31/22  185:28M
12/22/22  Ecovyst Inc.                      8-K:5,9    12/16/22   13:568K                                   Donnelley … Solutions/FA
 3/01/22  Ecovyst Inc.                      10-K       12/31/21  181:24M
 1/14/22  Ecovyst Inc.                      S-8         1/14/22    5:168K                                   Donnelley … Solutions/FA
11/09/21  Ecovyst Inc.                      10-Q        9/30/21  106:12M
 8/09/21  Ecovyst Inc.                      10-Q        6/30/21  104:12M
 8/03/21  Ecovyst Inc.                      8-K:1,2,5,8 8/01/21   12:265K                                   Donnelley … Solutions/FA
 7/15/21  Ecovyst Inc.                      8-K:1,9     7/12/21   11:212K                                   Donnelley … Solutions/FA
 6/30/21  Ecovyst Inc.                      8-K:1,9     6/24/21   11:149K                                   Donnelley … Solutions/FA
 6/11/21  Ecovyst Inc.                      8-K:1,2,9   6/09/21   12:5.8M                                   Donnelley … Solutions/FA
 3/04/21  Ecovyst Inc.                      8-K:1,7,8,9 2/28/21   12:978K                                   Donnelley … Solutions/FA
10/16/20  Ecovyst Inc.                      8-K:1,8,9  10/15/20   12:911K                                   Donnelley … Solutions/FA
 2/27/20  Ecovyst Inc.                      10-K       12/31/19  182:30M
12/13/17  Ecovyst Inc.                      8-K:1,2,9  12/11/17    2:929K                                   Donnelley … Solutions/FA
11/14/17  Ecovyst Inc.                      10-Q        9/30/17   92:9.3M
 9/01/17  Ecovyst Inc.                      S-1/A¶                15:9M                                     Donnelley … Solutions/FA
 8/14/17  Ecovyst Inc.                      S-1/A¶                12:9.2M                                   Donnelley … Solutions/FA
 6/09/17  Ecovyst Inc.                      S-1                   16:14M                                    Donnelley … Solutions/FA

↑Top

Filing Submission 0001708035-24-000070 – Alternative Formats (Word / Rich Text, HTML, Plain Text, et al.)

Ecovyst Inc. – ‘10-K’ for 12/31/23 – ‘EX-101.DEF’

On: Thursday, 2/29/24, at 4:02pm ET · For: 12/31/23 · Accession #: 1708035-24-70 · File #: 1-38221

Annual Report — Form 10-K — SEA’34Filing Table of Contents

‘EX-101.DEF’ — XBRL Definitions — ecvt-20231231_def

This Exhibit is an XBRL File.

18 Previous Filings that this Filing References

Annual Report — Form 10-K — SEA’34

Filing Table of Contents