<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019e6493-a735-7c87-a7f8-f29c0be1aa13,g:f88253c3-2347-42ed-9677-f1523f9486e7-->
<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.koppers.com/role/CONDENSEDCONSOLIDATEDSTATEMENTOFSHAREHOLDERSEQUITY" xlink:type="simple" xlink:href="kop-20260630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTOFSHAREHOLDERSEQUITY"/>
  <link:definitionLink xlink:role="http://www.koppers.com/role/CONDENSEDCONSOLIDATEDSTATEMENTOFSHAREHOLDERSEQUITY" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019e6493-a8b2-70a5-8841-04de2a1cf420" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019e6493-a8b2-7018-be87-f8ef079e7c04" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019e6493-a8b2-70a5-8841-04de2a1cf420" xlink:to="loc_us-gaap_StatementTable_019e6493-a8b2-7018-be87-f8ef079e7c04" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019e6493-a8b2-7612-8e9d-a93da01c3924" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e6493-a8b2-7018-be87-f8ef079e7c04" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019e6493-a8b2-7612-8e9d-a93da01c3924" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e6493-a8b2-7612-8e9d-a93da01c3924_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e6493-a8b2-7612-8e9d-a93da01c3924" xlink:to="loc_us-gaap_EquityComponentDomain_019e6493-a8b2-7612-8e9d-a93da01c3924_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e6493-a8b2-76de-ba6d-626c8f653a26" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e6493-a8b2-7612-8e9d-a93da01c3924" xlink:to="loc_us-gaap_EquityComponentDomain_019e6493-a8b2-76de-ba6d-626c8f653a26" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019e6493-a8b2-7c86-9ead-7a98b68d3bf6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e6493-a8b2-76de-ba6d-626c8f653a26" xlink:to="loc_us-gaap_CommonStockMember_019e6493-a8b2-7c86-9ead-7a98b68d3bf6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019e6493-a8b2-7628-b38d-0eaa1d5232df" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e6493-a8b2-76de-ba6d-626c8f653a26" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019e6493-a8b2-7628-b38d-0eaa1d5232df" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019e6493-a8b2-7f5d-9c27-490b1197a0e7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e6493-a8b2-76de-ba6d-626c8f653a26" xlink:to="loc_us-gaap_RetainedEarningsMember_019e6493-a8b2-7f5d-9c27-490b1197a0e7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e6493-a8b2-78de-8521-da17ac02ed7d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e6493-a8b2-76de-ba6d-626c8f653a26" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e6493-a8b2-78de-8521-da17ac02ed7d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonMember_019e6493-a8b2-7b2f-b1b8-05586f7057c5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockCommonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e6493-a8b2-76de-ba6d-626c8f653a26" xlink:to="loc_us-gaap_TreasuryStockCommonMember_019e6493-a8b2-7b2f-b1b8-05586f7057c5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncontrollingInterestMember_019e6493-a8b2-7cde-9fcb-97253c75e3a7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NoncontrollingInterestMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e6493-a8b2-76de-ba6d-626c8f653a26" xlink:to="loc_us-gaap_NoncontrollingInterestMember_019e6493-a8b2-7cde-9fcb-97253c75e3a7" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6493-a8b2-7693-8884-5aec3c5894c2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e6493-a8b2-70a5-8841-04de2a1cf420" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6493-a8b2-7693-8884-5aec3c5894c2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019e6493-a8b2-7541-9393-6684d676c027" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6493-a8b2-7693-8884-5aec3c5894c2" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019e6493-a8b2-7541-9393-6684d676c027" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019e6493-a8b2-7957-b6a6-1928d2c88823" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6493-a8b2-7693-8884-5aec3c5894c2" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019e6493-a8b2-7957-b6a6-1928d2c88823" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_019e6493-a8b2-72c0-ac02-4d55be3f73d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6493-a8b2-7693-8884-5aec3c5894c2" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_019e6493-a8b2-72c0-ac02-4d55be3f73d2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_019e6493-a8b2-7065-9ed4-c6e94e18bd2c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6493-a8b2-7693-8884-5aec3c5894c2" xlink:to="loc_us-gaap_ProfitLoss_019e6493-a8b2-7065-9ed4-c6e94e18bd2c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsCommonStock_019e6493-a8b2-7e2e-b3c2-fd9a23f65813" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DividendsCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6493-a8b2-7693-8884-5aec3c5894c2" xlink:to="loc_us-gaap_DividendsCommonStock_019e6493-a8b2-7e2e-b3c2-fd9a23f65813" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_ReturnOfCapitalToNoncontrollingInterests_019e6493-a8b2-7136-9012-ee2d35e21718" xlink:href="kop-20260630.xsd#kop_ReturnOfCapitalToNoncontrollingInterests"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6493-a8b2-7693-8884-5aec3c5894c2" xlink:to="loc_kop_ReturnOfCapitalToNoncontrollingInterests_019e6493-a8b2-7136-9012-ee2d35e21718" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_019e6493-a8b2-7948-8db6-b6d4a40b104e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6493-a8b2-7693-8884-5aec3c5894c2" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_019e6493-a8b2-7948-8db6-b6d4a40b104e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax_019e6493-a8b2-7119-ac0c-828e5c7ef9b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6493-a8b2-7693-8884-5aec3c5894c2" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax_019e6493-a8b2-7119-ac0c-828e5c7ef9b6" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax_019e6493-a8b2-777b-bfd2-721c7da291a3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6493-a8b2-7693-8884-5aec3c5894c2" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax_019e6493-a8b2-777b-bfd2-721c7da291a3" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_019e6493-a8b2-7cdb-95c1-19ee24183b5d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockValueAcquiredCostMethod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6493-a8b2-7693-8884-5aec3c5894c2" xlink:to="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_019e6493-a8b2-7cdb-95c1-19ee24183b5d" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019e6493-a8b2-7a6a-ae43-259d037e38cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_019e6493-a8b2-7d49-9900-b0ff74c74045" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6493-a8b2-7693-8884-5aec3c5894c2" xlink:to="loc_us-gaap_SharesOutstanding_019e6493-a8b2-7d49-9900-b0ff74c74045" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_019e6493-a8b2-7680-ab38-a7e94d4cd455" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6493-a8b2-7693-8884-5aec3c5894c2" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_019e6493-a8b2-7680-ab38-a7e94d4cd455" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_019e6493-a8b2-7ba0-9600-cbb406b68873" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonShares_019e6493-a8b2-74d5-9449-49f67d874a46" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockCommonShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6493-a8b2-7693-8884-5aec3c5894c2" xlink:to="loc_us-gaap_TreasuryStockCommonShares_019e6493-a8b2-74d5-9449-49f67d874a46" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockSharesAcquired_019e6493-a8b2-7037-9862-600d37c08b76" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockSharesAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e6493-a8b2-7693-8884-5aec3c5894c2" xlink:to="loc_us-gaap_TreasuryStockSharesAcquired_019e6493-a8b2-7037-9862-600d37c08b76" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonShares_019e6493-a8b2-735f-b9ef-7359497e426b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockCommonShares"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019e6493-a8b2-798d-bac4-5b81aea71623" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e6493-a8b2-70a5-8841-04de2a1cf420" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019e6493-a8b2-798d-bac4-5b81aea71623" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.koppers.com/role/RestructuringSummaryofRestructuringActivitiesDetails" xlink:type="simple" xlink:href="kop-20260630.xsd#RestructuringSummaryofRestructuringActivitiesDetails"/>
  <link:definitionLink xlink:role="http://www.koppers.com/role/RestructuringSummaryofRestructuringActivitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_019e6493-a8b2-73cc-82a1-53d34b31da7d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringCostAndReserveLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019e6493-a8b2-7d4e-918c-025320525739" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_019e6493-a8b2-73cc-82a1-53d34b31da7d" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019e6493-a8b2-7d4e-918c-025320525739" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_019e6493-a8b2-72e6-8523-edbbe4ce883b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringCostAndReserveAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019e6493-a8b2-7d4e-918c-025320525739" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_019e6493-a8b2-72e6-8523-edbbe4ce883b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_019e6493-a8b2-72e6-8523-edbbe4ce883b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_019e6493-a8b2-72e6-8523-edbbe4ce883b" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_019e6493-a8b2-72e6-8523-edbbe4ce883b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_019e6493-a8b2-7343-b2a9-25e16cbae01f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_019e6493-a8b2-72e6-8523-edbbe4ce883b" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_019e6493-a8b2-7343-b2a9-25e16cbae01f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeSeveranceMember_019e6493-a8b2-781f-9834-966d84af1703" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeSeveranceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_019e6493-a8b2-7343-b2a9-25e16cbae01f" xlink:to="loc_us-gaap_EmployeeSeveranceMember_019e6493-a8b2-781f-9834-966d84af1703" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_ImpairmentMember_019f6eda-aaaf-7ec7-b71b-05da9eac8604" xlink:href="kop-20260630.xsd#kop_ImpairmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_019e6493-a8b2-7343-b2a9-25e16cbae01f" xlink:to="loc_kop_ImpairmentMember_019f6eda-aaaf-7ec7-b71b-05da9eac8604" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_DepreciationAndAssetDisposalCostsMember_019e6493-a8b2-7300-9d4d-147e2efe0a14" xlink:href="kop-20260630.xsd#kop_DepreciationAndAssetDisposalCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_019e6493-a8b2-7343-b2a9-25e16cbae01f" xlink:to="loc_kop_DepreciationAndAssetDisposalCostsMember_019e6493-a8b2-7300-9d4d-147e2efe0a14" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_PlantCleaningWasteDisposalAndDemolitionCostsMember_019e6493-a8b2-7e49-ba2c-a0acfa7bf9c9" xlink:href="kop-20260630.xsd#kop_PlantCleaningWasteDisposalAndDemolitionCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_019e6493-a8b2-7343-b2a9-25e16cbae01f" xlink:to="loc_kop_PlantCleaningWasteDisposalAndDemolitionCostsMember_019e6493-a8b2-7e49-ba2c-a0acfa7bf9c9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_WorkforceReductionProgramMember_019e6493-a8b2-74dd-9762-809be178219b" xlink:href="kop-20260630.xsd#kop_WorkforceReductionProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_019e6493-a8b2-7343-b2a9-25e16cbae01f" xlink:to="loc_kop_WorkforceReductionProgramMember_019e6493-a8b2-74dd-9762-809be178219b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_ConsultingServicesMember_019e6493-a8b2-728f-9acd-7c329dc6c396" xlink:href="kop-20260630.xsd#kop_ConsultingServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_019e6493-a8b2-7343-b2a9-25e16cbae01f" xlink:to="loc_kop_ConsultingServicesMember_019e6493-a8b2-728f-9acd-7c329dc6c396" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherRestructuringMember_019e6493-a8b2-7e46-8efc-199c325f1d25" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherRestructuringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_019e6493-a8b2-7343-b2a9-25e16cbae01f" xlink:to="loc_us-gaap_OtherRestructuringMember_019e6493-a8b2-7e46-8efc-199c325f1d25" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanAxis_019e6493-a8b2-7477-b7ba-b9fc8cd3808d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringPlanAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019e6493-a8b2-7d4e-918c-025320525739" xlink:to="loc_us-gaap_RestructuringPlanAxis_019e6493-a8b2-7477-b7ba-b9fc8cd3808d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_019e6493-a8b2-7477-b7ba-b9fc8cd3808d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringPlanAxis_019e6493-a8b2-7477-b7ba-b9fc8cd3808d" xlink:to="loc_us-gaap_RestructuringPlanDomain_019e6493-a8b2-7477-b7ba-b9fc8cd3808d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_019e6493-a8b2-7446-83f2-43615e383e39" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringPlanAxis_019e6493-a8b2-7477-b7ba-b9fc8cd3808d" xlink:to="loc_us-gaap_RestructuringPlanDomain_019e6493-a8b2-7446-83f2-43615e383e39" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_StickneyShutdownMember_019edf77-fce3-7521-b77c-983ea826dc99" xlink:href="kop-20260630.xsd#kop_StickneyShutdownMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_019e6493-a8b2-7446-83f2-43615e383e39" xlink:to="loc_kop_StickneyShutdownMember_019edf77-fce3-7521-b77c-983ea826dc99" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_PhthalicAnhydrideShutdownMember_019e6493-a8b2-7b9e-b1cd-d8ccb94957d2" xlink:href="kop-20260630.xsd#kop_PhthalicAnhydrideShutdownMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_019e6493-a8b2-7446-83f2-43615e383e39" xlink:to="loc_kop_PhthalicAnhydrideShutdownMember_019e6493-a8b2-7b9e-b1cd-d8ccb94957d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostsAndAssetImpairmentCharges_019e6493-a8b2-7566-a496-1196a290b478" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringCostsAndAssetImpairmentCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_019e6493-a8b2-73cc-82a1-53d34b31da7d" xlink:to="loc_us-gaap_RestructuringCostsAndAssetImpairmentCharges_019e6493-a8b2-7566-a496-1196a290b478" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.koppers.com/role/RestructuringSummaryofPretaxChargesRestructuringActivitiesDetails" xlink:type="simple" xlink:href="kop-20260630.xsd#RestructuringSummaryofPretaxChargesRestructuringActivitiesDetails"/>
  <link:definitionLink xlink:role="http://www.koppers.com/role/RestructuringSummaryofPretaxChargesRestructuringActivitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_019fa362-d666-7828-a7f3-ee69a0749ee1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringCostAndReserveLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019fa362-d666-7ce1-af40-14aa36bea446" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_019fa362-d666-7828-a7f3-ee69a0749ee1" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019fa362-d666-7ce1-af40-14aa36bea446" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019fa362-d666-7b82-9186-1d91bbb87097" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019fa362-d666-7ce1-af40-14aa36bea446" xlink:to="loc_srt_RangeAxis_019fa362-d666-7b82-9186-1d91bbb87097" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019fa362-d666-7b82-9186-1d91bbb87097_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019fa362-d666-7b82-9186-1d91bbb87097" xlink:to="loc_srt_RangeMember_019fa362-d666-7b82-9186-1d91bbb87097_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019fa362-d666-7572-930c-fcae57a90f87" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019fa362-d666-7b82-9186-1d91bbb87097" xlink:to="loc_srt_RangeMember_019fa362-d666-7572-930c-fcae57a90f87" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019fa363-6c42-7889-94cf-b18123b644a0" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019fa362-d666-7572-930c-fcae57a90f87" xlink:to="loc_srt_MinimumMember_019fa363-6c42-7889-94cf-b18123b644a0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019fa363-93f8-7f76-b60d-53188a0eb4ba" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019fa362-d666-7572-930c-fcae57a90f87" xlink:to="loc_srt_MaximumMember_019fa363-93f8-7f76-b60d-53188a0eb4ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_019fa362-d666-7314-b62b-e0c03800497d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringCostAndReserveAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019fa362-d666-7ce1-af40-14aa36bea446" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_019fa362-d666-7314-b62b-e0c03800497d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_019fa362-d666-7314-b62b-e0c03800497d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_019fa362-d666-7314-b62b-e0c03800497d" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_019fa362-d666-7314-b62b-e0c03800497d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_019fa362-d666-7124-8153-93e5d5475e8c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_019fa362-d666-7314-b62b-e0c03800497d" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_019fa362-d666-7124-8153-93e5d5475e8c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_CashRestructuringCostsMember_019fd357-db90-7cd7-9cf7-f68074499b11" xlink:href="kop-20260630.xsd#kop_CashRestructuringCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_019fa362-d666-7124-8153-93e5d5475e8c" xlink:to="loc_kop_CashRestructuringCostsMember_019fd357-db90-7cd7-9cf7-f68074499b11" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeSeveranceMember_019fa362-d666-720c-92ac-25dfd4b35d34" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeSeveranceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_kop_CashRestructuringCostsMember_019fd357-db90-7cd7-9cf7-f68074499b11" xlink:to="loc_us-gaap_EmployeeSeveranceMember_019fa362-d666-720c-92ac-25dfd4b35d34" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_PlantCleaningWasteDisposalAndDemolitionCostsMember_019fa362-d666-779e-9570-c1b8727a9759" xlink:href="kop-20260630.xsd#kop_PlantCleaningWasteDisposalAndDemolitionCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_kop_CashRestructuringCostsMember_019fd357-db90-7cd7-9cf7-f68074499b11" xlink:to="loc_kop_PlantCleaningWasteDisposalAndDemolitionCostsMember_019fa362-d666-779e-9570-c1b8727a9759" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_NoncashRestructuringCostsMember_019fd354-d1d7-7736-8629-996ce7dbe4c5" xlink:href="kop-20260630.xsd#kop_NoncashRestructuringCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_019fa362-d666-7124-8153-93e5d5475e8c" xlink:to="loc_kop_NoncashRestructuringCostsMember_019fd354-d1d7-7736-8629-996ce7dbe4c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_ImpairmentDepreciationAndAssetDisposalCostsMember_019fa362-d666-70df-8fcd-3dbed392939d" xlink:href="kop-20260630.xsd#kop_ImpairmentDepreciationAndAssetDisposalCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_kop_NoncashRestructuringCostsMember_019fd354-d1d7-7736-8629-996ce7dbe4c5" xlink:to="loc_kop_ImpairmentDepreciationAndAssetDisposalCostsMember_019fa362-d666-70df-8fcd-3dbed392939d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_DepreciationAndAssetDisposalCostsMember_019fa362-d666-78e6-84ee-dcd31bcd9490" xlink:href="kop-20260630.xsd#kop_DepreciationAndAssetDisposalCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_kop_NoncashRestructuringCostsMember_019fd354-d1d7-7736-8629-996ce7dbe4c5" xlink:to="loc_kop_DepreciationAndAssetDisposalCostsMember_019fa362-d666-78e6-84ee-dcd31bcd9490" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanAxis_019fa362-d666-703b-bdb9-2366719bc9e6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringPlanAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019fa362-d666-7ce1-af40-14aa36bea446" xlink:to="loc_us-gaap_RestructuringPlanAxis_019fa362-d666-703b-bdb9-2366719bc9e6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_019fa362-d666-703b-bdb9-2366719bc9e6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringPlanAxis_019fa362-d666-703b-bdb9-2366719bc9e6" xlink:to="loc_us-gaap_RestructuringPlanDomain_019fa362-d666-703b-bdb9-2366719bc9e6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_019fa362-d666-739f-a9c6-66b80749b64c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringPlanAxis_019fa362-d666-703b-bdb9-2366719bc9e6" xlink:to="loc_us-gaap_RestructuringPlanDomain_019fa362-d666-739f-a9c6-66b80749b64c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_StickneyShutdownMember_019fa362-d666-74c0-b889-8ccdfe0a2933" xlink:href="kop-20260630.xsd#kop_StickneyShutdownMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_019fa362-d666-739f-a9c6-66b80749b64c" xlink:to="loc_kop_StickneyShutdownMember_019fa362-d666-74c0-b889-8ccdfe0a2933" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_PhthalicAnhydrideShutdownMember_019fa362-d666-7792-b0be-cafb59f84a55" xlink:href="kop-20260630.xsd#kop_PhthalicAnhydrideShutdownMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_019fa362-d666-739f-a9c6-66b80749b64c" xlink:to="loc_kop_PhthalicAnhydrideShutdownMember_019fa362-d666-7792-b0be-cafb59f84a55" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringAndRelatedCostExpectedCost1_019fa37c-31aa-7d27-a0ee-1862de5371ec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringAndRelatedCostExpectedCost1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_019fa362-d666-7828-a7f3-ee69a0749ee1" xlink:to="loc_us-gaap_RestructuringAndRelatedCostExpectedCost1_019fa37c-31aa-7d27-a0ee-1862de5371ec" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.koppers.com/role/RestructuringSummaryofDetailsofPlantClosuresandRestructuringLiabilitiesDetails" xlink:type="simple" xlink:href="kop-20260630.xsd#RestructuringSummaryofDetailsofPlantClosuresandRestructuringLiabilitiesDetails"/>
  <link:definitionLink xlink:role="http://www.koppers.com/role/RestructuringSummaryofDetailsofPlantClosuresandRestructuringLiabilitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_019e6493-a8b2-7fa8-9650-9dbf0ea55e55" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringCostAndReserveLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019e6493-a8b2-7b67-8f19-c0fb3e756a68" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_019e6493-a8b2-7fa8-9650-9dbf0ea55e55" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019e6493-a8b2-7b67-8f19-c0fb3e756a68" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanAxis_019e6493-a8b2-769a-b97a-d0e91c0b89c6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringPlanAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019e6493-a8b2-7b67-8f19-c0fb3e756a68" xlink:to="loc_us-gaap_RestructuringPlanAxis_019e6493-a8b2-769a-b97a-d0e91c0b89c6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_019e6493-a8b2-769a-b97a-d0e91c0b89c6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringPlanAxis_019e6493-a8b2-769a-b97a-d0e91c0b89c6" xlink:to="loc_us-gaap_RestructuringPlanDomain_019e6493-a8b2-769a-b97a-d0e91c0b89c6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_019e6493-a8b2-7a0a-b9e6-d7c39ce753d7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringPlanAxis_019e6493-a8b2-769a-b97a-d0e91c0b89c6" xlink:to="loc_us-gaap_RestructuringPlanDomain_019e6493-a8b2-7a0a-b9e6-d7c39ce753d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_StickneyShutdownMember_019edf81-456e-7572-b1cf-01c2c1082e27" xlink:href="kop-20260630.xsd#kop_StickneyShutdownMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_019e6493-a8b2-7a0a-b9e6-d7c39ce753d7" xlink:to="loc_kop_StickneyShutdownMember_019edf81-456e-7572-b1cf-01c2c1082e27" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_PhthalicAnhydrideShutdownMember_019e6493-a8b2-7508-bdbf-8dde83982553" xlink:href="kop-20260630.xsd#kop_PhthalicAnhydrideShutdownMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_019e6493-a8b2-7a0a-b9e6-d7c39ce753d7" xlink:to="loc_kop_PhthalicAnhydrideShutdownMember_019e6493-a8b2-7508-bdbf-8dde83982553" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_019e6493-a8b2-7452-8716-28b6a0e613e4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringCostAndReserveAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019e6493-a8b2-7b67-8f19-c0fb3e756a68" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_019e6493-a8b2-7452-8716-28b6a0e613e4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_019e6493-a8b2-7452-8716-28b6a0e613e4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_019e6493-a8b2-7452-8716-28b6a0e613e4" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_019e6493-a8b2-7452-8716-28b6a0e613e4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_019e6493-a8b2-79e2-a488-207908d9ef8c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_019e6493-a8b2-7452-8716-28b6a0e613e4" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_019e6493-a8b2-79e2-a488-207908d9ef8c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_WorkforceReductionProgramMember_019e6493-a8b2-77fc-99dd-b117ecfe38a8" xlink:href="kop-20260630.xsd#kop_WorkforceReductionProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_019e6493-a8b2-79e2-a488-207908d9ef8c" xlink:to="loc_kop_WorkforceReductionProgramMember_019e6493-a8b2-77fc-99dd-b117ecfe38a8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserveRollForward_019e6493-a8b2-741a-90ff-20be13fc9de8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringReserveRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_019e6493-a8b2-7fa8-9650-9dbf0ea55e55" xlink:to="loc_us-gaap_RestructuringReserveRollForward_019e6493-a8b2-741a-90ff-20be13fc9de8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserve_019e6493-a8b2-764d-91c4-99f318944be9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringReserve"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_019e6493-a8b2-741a-90ff-20be13fc9de8" xlink:to="loc_us-gaap_RestructuringReserve_019e6493-a8b2-764d-91c4-99f318944be9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserveAccrualAdjustment1_019e6493-a8b2-75c9-afcf-75df7a91511d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringReserveAccrualAdjustment1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_019e6493-a8b2-741a-90ff-20be13fc9de8" xlink:to="loc_us-gaap_RestructuringReserveAccrualAdjustment1_019e6493-a8b2-75c9-afcf-75df7a91511d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForRestructuring_019e6493-a8b2-70d2-b41e-cced305b9cd6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsForRestructuring"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_019e6493-a8b2-741a-90ff-20be13fc9de8" xlink:to="loc_us-gaap_PaymentsForRestructuring_019e6493-a8b2-70d2-b41e-cced305b9cd6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserve_019e6493-a8b2-79ee-8efd-906e0f5424ff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringReserve"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetRetirementObligationCurrent_019fa382-78b6-7e05-8341-af11a78a3eb2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetRetirementObligationCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_019e6493-a8b2-7fa8-9650-9dbf0ea55e55" xlink:to="loc_us-gaap_AssetRetirementObligationCurrent_019fa382-78b6-7e05-8341-af11a78a3eb2" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.koppers.com/role/FairValueMeasurementsDetails" xlink:type="simple" xlink:href="kop-20260630.xsd#FairValueMeasurementsDetails"/>
  <link:definitionLink xlink:role="http://www.koppers.com/role/FairValueMeasurementsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019e6493-a8b2-71f5-aebb-9333b4a479b9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019e6493-a8b2-763c-a347-075701c8a29e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByBalanceSheetGroupingTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019e6493-a8b2-71f5-aebb-9333b4a479b9" xlink:to="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019e6493-a8b2-763c-a347-075701c8a29e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementBasisAxis_019e6493-a8b2-73f6-9b25-510234d3a40a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByMeasurementBasisAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019e6493-a8b2-763c-a347-075701c8a29e" xlink:to="loc_us-gaap_FairValueByMeasurementBasisAxis_019e6493-a8b2-73f6-9b25-510234d3a40a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019e6493-a8b2-73f6-9b25-510234d3a40a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_019e6493-a8b2-73f6-9b25-510234d3a40a" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019e6493-a8b2-73f6-9b25-510234d3a40a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019e6493-a8b2-7dcf-b54f-5656d5f90227" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueDisclosureItemAmountsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_019e6493-a8b2-73f6-9b25-510234d3a40a" xlink:to="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019e6493-a8b2-7dcf-b54f-5656d5f90227" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019e6493-a8b2-7fbe-9941-2b8b9c033528" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019e6493-a8b2-7dcf-b54f-5656d5f90227" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019e6493-a8b2-7fbe-9941-2b8b9c033528" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_019e6493-a8b2-7cd5-a9e4-c4ba63efafe9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019e6493-a8b2-7fbe-9941-2b8b9c033528" xlink:to="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_019e6493-a8b2-7cd5-a9e4-c4ba63efafe9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_019e6493-a8b2-7143-b19b-d442c51e6f4a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CarryingReportedAmountFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019e6493-a8b2-7fbe-9941-2b8b9c033528" xlink:to="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_019e6493-a8b2-7143-b19b-d442c51e6f4a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract_019e6493-a8b2-7549-a33f-7067b75b23e8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019e6493-a8b2-71f5-aebb-9333b4a479b9" xlink:to="loc_us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract_019e6493-a8b2-7549-a33f-7067b75b23e8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsFairValueDisclosure_019e6493-a8b2-774e-9315-1f162969b716" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherAssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract_019e6493-a8b2-7549-a33f-7067b75b23e8" xlink:to="loc_us-gaap_OtherAssetsFairValueDisclosure_019e6493-a8b2-774e-9315-1f162969b716" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract_019e6493-a8b2-7af1-9472-48cf86957d9d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019e6493-a8b2-71f5-aebb-9333b4a479b9" xlink:to="loc_us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract_019e6493-a8b2-7af1-9472-48cf86957d9d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFairValue_019e6493-a8b2-7233-a097-d4c94eb6f627" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract_019e6493-a8b2-7af1-9472-48cf86957d9d" xlink:to="loc_us-gaap_DebtInstrumentFairValue_019e6493-a8b2-7233-a097-d4c94eb6f627" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.koppers.com/role/DerivativeFinancialInstrumentsAdditionalInformationDetails" xlink:type="simple" xlink:href="kop-20260630.xsd#DerivativeFinancialInstrumentsAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.koppers.com/role/DerivativeFinancialInstrumentsAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLineItems_019e6493-a8b2-757f-aab0-de78d07da19c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeTable_019e6493-a8b2-751e-b2e8-29f59eeca7e0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeLineItems_019e6493-a8b2-757f-aab0-de78d07da19c" xlink:to="loc_us-gaap_DerivativeTable_019e6493-a8b2-751e-b2e8-29f59eeca7e0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019e6493-a8b2-7487-bdc5-09f4a15ed253" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_019e6493-a8b2-751e-b2e8-29f59eeca7e0" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019e6493-a8b2-7487-bdc5-09f4a15ed253" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019e6493-a8b2-7487-bdc5-09f4a15ed253_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019e6493-a8b2-7487-bdc5-09f4a15ed253" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019e6493-a8b2-7487-bdc5-09f4a15ed253_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019e6493-a8b2-77fa-8ab3-0c601c08e0f2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019e6493-a8b2-7487-bdc5-09f4a15ed253" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019e6493-a8b2-77fa-8ab3-0c601c08e0f2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateSwapMember_019e6493-a8b2-7ecd-b71d-ffce4b9fd42c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestRateSwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019e6493-a8b2-77fa-8ab3-0c601c08e0f2" xlink:to="loc_us-gaap_InterestRateSwapMember_019e6493-a8b2-7ecd-b71d-ffce4b9fd42c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_InterestRateSwapOneMember_019f6f59-43f9-732c-9777-9e41e057ac6a" xlink:href="kop-20260630.xsd#kop_InterestRateSwapOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019e6493-a8b2-77fa-8ab3-0c601c08e0f2" xlink:to="loc_kop_InterestRateSwapOneMember_019f6f59-43f9-732c-9777-9e41e057ac6a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_InterestRateSwapTwoMember_019f6f59-8cf1-749b-8ddb-9173eb42a231" xlink:href="kop-20260630.xsd#kop_InterestRateSwapTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019e6493-a8b2-77fa-8ab3-0c601c08e0f2" xlink:to="loc_kop_InterestRateSwapTwoMember_019f6f59-8cf1-749b-8ddb-9173eb42a231" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_InterestRateSwapThreeMember_019fa328-e6c2-7922-b70c-715b29bb53ae" xlink:href="kop-20260630.xsd#kop_InterestRateSwapThreeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019e6493-a8b2-77fa-8ab3-0c601c08e0f2" xlink:to="loc_kop_InterestRateSwapThreeMember_019fa328-e6c2-7922-b70c-715b29bb53ae" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommodityContractMember_019e6493-a8b2-7f65-864e-7f3dc5131024" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommodityContractMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019e6493-a8b2-77fa-8ab3-0c601c08e0f2" xlink:to="loc_us-gaap_CommodityContractMember_019e6493-a8b2-7f65-864e-7f3dc5131024" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MaximumLengthOfTimeHedgedInCashFlowHedge1_019e6493-a8b2-7ca2-9d06-6ffdd1bc878a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MaximumLengthOfTimeHedgedInCashFlowHedge1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019e6493-a8b2-757f-aab0-de78d07da19c" xlink:to="loc_us-gaap_MaximumLengthOfTimeHedgedInCashFlowHedge1_019e6493-a8b2-7ca2-9d06-6ffdd1bc878a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossReclassificationFromAccumulatedOCIToIncomeEstimatedNetAmountToBeTransferred_019e6493-a8b2-724c-a69a-00496b7f0af3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentsGainLossReclassificationFromAccumulatedOCIToIncomeEstimatedNetAmountToBeTransferred"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019e6493-a8b2-757f-aab0-de78d07da19c" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossReclassificationFromAccumulatedOCIToIncomeEstimatedNetAmountToBeTransferred_019e6493-a8b2-724c-a69a-00496b7f0af3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeNonmonetaryNotionalAmountVolume_019e6493-a8b2-785e-be3b-622dc8ce2a7e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeNonmonetaryNotionalAmountVolume"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019e6493-a8b2-757f-aab0-de78d07da19c" xlink:to="loc_us-gaap_DerivativeNonmonetaryNotionalAmountVolume_019e6493-a8b2-785e-be3b-622dc8ce2a7e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeNotionalAmount_019e6493-a8b2-7e55-a357-1c951be71491" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeNotionalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019e6493-a8b2-757f-aab0-de78d07da19c" xlink:to="loc_us-gaap_DerivativeNotionalAmount_019e6493-a8b2-7e55-a357-1c951be71491" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeFixedInterestRate_019e6493-a8b2-7df5-b481-4d07ec86d497" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeFixedInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019e6493-a8b2-757f-aab0-de78d07da19c" xlink:to="loc_us-gaap_DerivativeFixedInterestRate_019e6493-a8b2-7df5-b481-4d07ec86d497" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossReclassificationFromAccumulatedOCIToIncomeEstimateOfTimeToTransfer1_019e6493-a8b2-7983-8790-50d9e2bff588" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentsGainLossReclassificationFromAccumulatedOCIToIncomeEstimateOfTimeToTransfer1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019e6493-a8b2-757f-aab0-de78d07da19c" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossReclassificationFromAccumulatedOCIToIncomeEstimateOfTimeToTransfer1_019e6493-a8b2-7983-8790-50d9e2bff588" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.koppers.com/role/DerivativeFinancialInstrumentsSummaryofFairValueofOutstandingDerivativeContractsRecordedinBalanceSheetDetails" xlink:type="simple" xlink:href="kop-20260630.xsd#DerivativeFinancialInstrumentsSummaryofFairValueofOutstandingDerivativeContractsRecordedinBalanceSheetDetails"/>
  <link:definitionLink xlink:role="http://www.koppers.com/role/DerivativeFinancialInstrumentsSummaryofFairValueofOutstandingDerivativeContractsRecordedinBalanceSheetDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLineItems_019e6493-a8b2-7682-968e-57ddee922545" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeTable_019e6493-a8b2-7fc5-8b6d-fbd74f13b8ab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeLineItems_019e6493-a8b2-7682-968e-57ddee922545" xlink:to="loc_us-gaap_DerivativeTable_019e6493-a8b2-7fc5-8b6d-fbd74f13b8ab" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019e6493-a8b2-7cf3-9bd6-f6d43c3dac49" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_019e6493-a8b2-7fc5-8b6d-fbd74f13b8ab" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019e6493-a8b2-7cf3-9bd6-f6d43c3dac49" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-7cf3-9bd6-f6d43c3dac49_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e6493-a8b2-7cf3-9bd6-f6d43c3dac49" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-7cf3-9bd6-f6d43c3dac49_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-70fc-ade2-0d14e36e3eef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e6493-a8b2-7cf3-9bd6-f6d43c3dac49" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-70fc-ade2-0d14e36e3eef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SwapMember_019e6493-a8b2-7ac6-9fd8-54aa335b6ca5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-70fc-ade2-0d14e36e3eef" xlink:to="loc_us-gaap_SwapMember_019e6493-a8b2-7ac6-9fd8-54aa335b6ca5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_HeatingOilContractsMember_019e6493-a8b2-75df-9bd4-01f4fcb394c1" xlink:href="kop-20260630.xsd#kop_HeatingOilContractsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-70fc-ade2-0d14e36e3eef" xlink:to="loc_kop_HeatingOilContractsMember_019e6493-a8b2-75df-9bd4-01f4fcb394c1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForwardContractsMember_019e6493-a8b2-7750-b876-0ff7ffad6b9e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForwardContractsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-70fc-ade2-0d14e36e3eef" xlink:to="loc_us-gaap_ForwardContractsMember_019e6493-a8b2-7750-b876-0ff7ffad6b9e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateSwapMember_019e6493-a8b2-72ea-a589-fadfb5bf206a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestRateSwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-70fc-ade2-0d14e36e3eef" xlink:to="loc_us-gaap_InterestRateSwapMember_019e6493-a8b2-72ea-a589-fadfb5bf206a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfFinancialPositionLocationBalanceAxis_019e6493-a8b2-78f0-9003-2280e44e60b2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOfFinancialPositionLocationBalanceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_019e6493-a8b2-7fc5-8b6d-fbd74f13b8ab" xlink:to="loc_us-gaap_StatementOfFinancialPositionLocationBalanceAxis_019e6493-a8b2-78f0-9003-2280e44e60b2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateDerivativesAtFairValueNet_019e6493-a8b2-74ce-9210-7262995837b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestRateDerivativesAtFairValueNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019e6493-a8b2-7682-968e-57ddee922545" xlink:to="loc_us-gaap_InterestRateDerivativesAtFairValueNet_019e6493-a8b2-74ce-9210-7262995837b3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AociLossCashFlowHedgeCumulativeGainLossAfterTax_019e6493-a8b2-76ca-9a4d-10b29a3a1422" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AociLossCashFlowHedgeCumulativeGainLossAfterTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019e6493-a8b2-7682-968e-57ddee922545" xlink:to="loc_us-gaap_AociLossCashFlowHedgeCumulativeGainLossAfterTax_019e6493-a8b2-76ca-9a4d-10b29a3a1422" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.koppers.com/role/DerivativeFinancialInstrumentsSummaryofFairValueofOutstandingDerivativeContractsRecordedinBalanceSheetDetails_1" xlink:type="simple" xlink:href="kop-20260630.xsd#DerivativeFinancialInstrumentsSummaryofFairValueofOutstandingDerivativeContractsRecordedinBalanceSheetDetails_1"/>
  <link:definitionLink xlink:role="http://www.koppers.com/role/DerivativeFinancialInstrumentsSummaryofFairValueofOutstandingDerivativeContractsRecordedinBalanceSheetDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLineItems_019e6493-a8b2-7682-968e-57ddee922545" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeTable_019e6493-a8b2-7fc5-8b6d-fbd74f13b8ab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeLineItems_019e6493-a8b2-7682-968e-57ddee922545" xlink:to="loc_us-gaap_DerivativeTable_019e6493-a8b2-7fc5-8b6d-fbd74f13b8ab" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019e6493-a8b2-7cf3-9bd6-f6d43c3dac49" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_019e6493-a8b2-7fc5-8b6d-fbd74f13b8ab" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019e6493-a8b2-7cf3-9bd6-f6d43c3dac49" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-7cf3-9bd6-f6d43c3dac49_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e6493-a8b2-7cf3-9bd6-f6d43c3dac49" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-7cf3-9bd6-f6d43c3dac49_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-70fc-ade2-0d14e36e3eef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e6493-a8b2-7cf3-9bd6-f6d43c3dac49" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-70fc-ade2-0d14e36e3eef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SwapMember_019e6493-a8b2-7ac6-9fd8-54aa335b6ca5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-70fc-ade2-0d14e36e3eef" xlink:to="loc_us-gaap_SwapMember_019e6493-a8b2-7ac6-9fd8-54aa335b6ca5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_HeatingOilContractsMember_019e6493-a8b2-75df-9bd4-01f4fcb394c1" xlink:href="kop-20260630.xsd#kop_HeatingOilContractsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-70fc-ade2-0d14e36e3eef" xlink:to="loc_kop_HeatingOilContractsMember_019e6493-a8b2-75df-9bd4-01f4fcb394c1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForwardContractsMember_019e6493-a8b2-7750-b876-0ff7ffad6b9e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForwardContractsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-70fc-ade2-0d14e36e3eef" xlink:to="loc_us-gaap_ForwardContractsMember_019e6493-a8b2-7750-b876-0ff7ffad6b9e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateSwapMember_019e6493-a8b2-72ea-a589-fadfb5bf206a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestRateSwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-70fc-ade2-0d14e36e3eef" xlink:to="loc_us-gaap_InterestRateSwapMember_019e6493-a8b2-72ea-a589-fadfb5bf206a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateDerivativesAtFairValueNet_019e6493-a8b2-74ce-9210-7262995837b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestRateDerivativesAtFairValueNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019e6493-a8b2-7682-968e-57ddee922545" xlink:to="loc_us-gaap_InterestRateDerivativesAtFairValueNet_019e6493-a8b2-74ce-9210-7262995837b3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AociLossCashFlowHedgeCumulativeGainLossAfterTax_019e6493-a8b2-76ca-9a4d-10b29a3a1422" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AociLossCashFlowHedgeCumulativeGainLossAfterTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019e6493-a8b2-7682-968e-57ddee922545" xlink:to="loc_us-gaap_AociLossCashFlowHedgeCumulativeGainLossAfterTax_019e6493-a8b2-76ca-9a4d-10b29a3a1422" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.koppers.com/role/DerivativeFinancialInstrumentsSummaryofUnrealizedGainLossfromHedgingContractsWhereHedgeAccountingWasNotElectedDetails" xlink:type="simple" xlink:href="kop-20260630.xsd#DerivativeFinancialInstrumentsSummaryofUnrealizedGainLossfromHedgingContractsWhereHedgeAccountingWasNotElectedDetails"/>
  <link:definitionLink xlink:role="http://www.koppers.com/role/DerivativeFinancialInstrumentsSummaryofUnrealizedGainLossfromHedgingContractsWhereHedgeAccountingWasNotElectedDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLineItems_019e6493-a8b2-7dff-9465-26f5214123ad" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeTable_019e6493-a8b2-72ae-8294-26dd0f8a4f30" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeLineItems_019e6493-a8b2-7dff-9465-26f5214123ad" xlink:to="loc_us-gaap_DerivativeTable_019e6493-a8b2-72ae-8294-26dd0f8a4f30" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019e6493-a8b2-7a34-9f45-5197e1cfcff8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_019e6493-a8b2-72ae-8294-26dd0f8a4f30" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019e6493-a8b2-7a34-9f45-5197e1cfcff8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-7a34-9f45-5197e1cfcff8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e6493-a8b2-7a34-9f45-5197e1cfcff8" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-7a34-9f45-5197e1cfcff8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-7a08-9789-0be24e6c3baf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e6493-a8b2-7a34-9f45-5197e1cfcff8" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-7a08-9789-0be24e6c3baf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SwapMember_019e6493-a8b2-7167-b9ba-23ccc24bc1dc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-7a08-9789-0be24e6c3baf" xlink:to="loc_us-gaap_SwapMember_019e6493-a8b2-7167-b9ba-23ccc24bc1dc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_HeatingOilContractsMember_019e6493-a8b2-71d6-9442-056c15cb0f7c" xlink:href="kop-20260630.xsd#kop_HeatingOilContractsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-7a08-9789-0be24e6c3baf" xlink:to="loc_kop_HeatingOilContractsMember_019e6493-a8b2-71d6-9442-056c15cb0f7c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForwardContractsMember_019e6493-a8b2-71dc-a8fc-088e96b7aa27" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForwardContractsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-7a08-9789-0be24e6c3baf" xlink:to="loc_us-gaap_ForwardContractsMember_019e6493-a8b2-71dc-a8fc-088e96b7aa27" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_019e6493-a8b2-74ff-809b-dddeafca56b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_019e6493-a8b2-72ae-8294-26dd0f8a4f30" xlink:to="loc_us-gaap_HedgingDesignationAxis_019e6493-a8b2-74ff-809b-dddeafca56b3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019e6493-a8b2-74ff-809b-dddeafca56b3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_019e6493-a8b2-74ff-809b-dddeafca56b3" xlink:to="loc_us-gaap_HedgingDesignationDomain_019e6493-a8b2-74ff-809b-dddeafca56b3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019e6493-a8b2-774f-b64a-e80abbf89e6d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_019e6493-a8b2-74ff-809b-dddeafca56b3" xlink:to="loc_us-gaap_HedgingDesignationDomain_019e6493-a8b2-774f-b64a-e80abbf89e6d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NondesignatedMember_019e6493-a8b2-7f26-b93c-d94681ff65b2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NondesignatedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_019e6493-a8b2-774f-b64a-e80abbf89e6d" xlink:to="loc_us-gaap_NondesignatedMember_019e6493-a8b2-7f26-b93c-d94681ff65b2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet_019e6493-a8b2-719a-8a84-0ba387cac17d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_019e6493-a8b2-7dff-9465-26f5214123ad" xlink:to="loc_us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet_019e6493-a8b2-719a-8a84-0ba387cac17d" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.koppers.com/role/DerivativeFinancialInstrumentsSummaryofOutstandingCopperSwapContractsDetails" xlink:type="simple" xlink:href="kop-20260630.xsd#DerivativeFinancialInstrumentsSummaryofOutstandingCopperSwapContractsDetails"/>
  <link:definitionLink xlink:role="http://www.koppers.com/role/DerivativeFinancialInstrumentsSummaryofOutstandingCopperSwapContractsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativesFairValueLineItems_019e6493-a8b2-794e-8e8d-596bb481626d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativesFairValueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019e6493-a8b2-7cbd-9f53-3d9493fe0b30" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019e6493-a8b2-794e-8e8d-596bb481626d" xlink:to="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019e6493-a8b2-7cbd-9f53-3d9493fe0b30" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019e6493-a8b2-799f-b495-045d51856f91" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019e6493-a8b2-7cbd-9f53-3d9493fe0b30" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019e6493-a8b2-799f-b495-045d51856f91" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-799f-b495-045d51856f91_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e6493-a8b2-799f-b495-045d51856f91" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-799f-b495-045d51856f91_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-7124-a9b9-4d6444580861" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e6493-a8b2-799f-b495-045d51856f91" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-7124-a9b9-4d6444580861" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SwapMember_019e6493-a8b2-7f27-ab45-9abf04dd0c32" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e6493-a8b2-7124-a9b9-4d6444580861" xlink:to="loc_us-gaap_SwapMember_019e6493-a8b2-7f27-ab45-9abf04dd0c32" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_019e6493-a8b2-7caa-a97a-1c1a29c418c2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019e6493-a8b2-7cbd-9f53-3d9493fe0b30" xlink:to="loc_us-gaap_HedgingDesignationAxis_019e6493-a8b2-7caa-a97a-1c1a29c418c2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019e6493-a8b2-7caa-a97a-1c1a29c418c2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_019e6493-a8b2-7caa-a97a-1c1a29c418c2" xlink:to="loc_us-gaap_HedgingDesignationDomain_019e6493-a8b2-7caa-a97a-1c1a29c418c2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_019e6493-a8b2-7bc9-a00e-11ebdac659d7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_019e6493-a8b2-7caa-a97a-1c1a29c418c2" xlink:to="loc_us-gaap_HedgingDesignationDomain_019e6493-a8b2-7bc9-a00e-11ebdac659d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NondesignatedMember_019e6493-a8b2-778a-8479-9f84d13dbbbc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NondesignatedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_019e6493-a8b2-7bc9-a00e-11ebdac659d7" xlink:to="loc_us-gaap_NondesignatedMember_019e6493-a8b2-778a-8479-9f84d13dbbbc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilityNotionalAmount_019e6493-a8b2-7055-9d59-a796d5547fbf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLiabilityNotionalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019e6493-a8b2-794e-8e8d-596bb481626d" xlink:to="loc_us-gaap_DerivativeLiabilityNotionalAmount_019e6493-a8b2-7055-9d59-a796d5547fbf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeNet_019e6493-a8b2-7636-8824-061c44622a63" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeFairValueOfDerivativeNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019e6493-a8b2-794e-8e8d-596bb481626d" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeNet_019e6493-a8b2-7636-8824-061c44622a63" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.koppers.com/role/EarningsandDividendsperCommonShareAdditionalInformationDetails" xlink:type="simple" xlink:href="kop-20260630.xsd#EarningsandDividendsperCommonShareAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.koppers.com/role/EarningsandDividendsperCommonShareAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019f6ef1-0824-7d16-b3ec-3f33b942f892" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019f6ef1-0824-7a8f-bbd6-4ba656c9d59f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019f6ef1-0824-7d16-b3ec-3f33b942f892" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019f6ef1-0824-7a8f-bbd6-4ba656c9d59f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019f6ef1-0824-7e00-8388-778f749d0ca8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019f6ef1-0824-7a8f-bbd6-4ba656c9d59f" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_019f6ef1-0824-7e00-8388-778f749d0ca8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019f6ef1-0824-7e00-8388-778f749d0ca8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019f6ef1-0824-7e00-8388-778f749d0ca8" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019f6ef1-0824-7e00-8388-778f749d0ca8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019f6ef1-0824-72f6-9c52-73e77a60e652" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019f6ef1-0824-7e00-8388-778f749d0ca8" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019f6ef1-0824-72f6-9c52-73e77a60e652" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_019f6ef1-0824-7aac-b01b-97db1cdbb402" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_019f6ef1-0824-72f6-9c52-73e77a60e652" xlink:to="loc_us-gaap_SubsequentEventMember_019f6ef1-0824-7aac-b01b-97db1cdbb402" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockDividendsPerShareDeclared_019f6ef1-0824-7428-b0a4-8ac51436670a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockDividendsPerShareDeclared"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019f6ef1-0824-7d16-b3ec-3f33b942f892" xlink:to="loc_us-gaap_CommonStockDividendsPerShareDeclared_019f6ef1-0824-7428-b0a4-8ac51436670a" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.koppers.com/role/StockbasedCompensationAdditionalInformationDetails" xlink:type="simple" xlink:href="kop-20260630.xsd#StockbasedCompensationAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.koppers.com/role/StockbasedCompensationAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6493-a8b2-7630-a125-cbf7a1ff59bc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e6493-a8b2-76ee-81c0-c8f1f43d15e6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6493-a8b2-7630-a125-cbf7a1ff59bc" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e6493-a8b2-76ee-81c0-c8f1f43d15e6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e6493-a8b2-7d86-871c-1e695b7b83fc" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e6493-a8b2-76ee-81c0-c8f1f43d15e6" xlink:to="loc_srt_RangeAxis_019e6493-a8b2-7d86-871c-1e695b7b83fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e6493-a8b2-7d86-871c-1e695b7b83fc_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e6493-a8b2-7d86-871c-1e695b7b83fc" xlink:to="loc_srt_RangeMember_019e6493-a8b2-7d86-871c-1e695b7b83fc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e6493-a8b2-71c9-b56a-47568da7eba5" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e6493-a8b2-7d86-871c-1e695b7b83fc" xlink:to="loc_srt_RangeMember_019e6493-a8b2-71c9-b56a-47568da7eba5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e6493-a8b2-7d4b-abc3-8fc451225609" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e6493-a8b2-71c9-b56a-47568da7eba5" xlink:to="loc_srt_MinimumMember_019e6493-a8b2-7d4b-abc3-8fc451225609" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e6493-a8b2-7b0f-a183-efe0545ae48f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e6493-a8b2-71c9-b56a-47568da7eba5" xlink:to="loc_srt_MaximumMember_019e6493-a8b2-7b0f-a183-efe0545ae48f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_WeightedAverageMember_019e6493-a8b2-7109-88da-830d0309af09" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_WeightedAverageMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e6493-a8b2-71c9-b56a-47568da7eba5" xlink:to="loc_srt_WeightedAverageMember_019e6493-a8b2-7109-88da-830d0309af09" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e6493-a8b2-709c-8271-d186b3929c84" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e6493-a8b2-76ee-81c0-c8f1f43d15e6" xlink:to="loc_us-gaap_AwardTypeAxis_019e6493-a8b2-709c-8271-d186b3929c84" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6493-a8b2-709c-8271-d186b3929c84_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e6493-a8b2-709c-8271-d186b3929c84" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6493-a8b2-709c-8271-d186b3929c84_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6493-a8b2-725d-b32a-b7bd13b1b26d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e6493-a8b2-709c-8271-d186b3929c84" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6493-a8b2-725d-b32a-b7bd13b1b26d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e6493-a8b2-7436-ac4f-dc6e0043cdcc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6493-a8b2-725d-b32a-b7bd13b1b26d" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e6493-a8b2-7436-ac4f-dc6e0043cdcc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019e6493-a8b2-7d54-858e-608cd138dc4c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6493-a8b2-725d-b32a-b7bd13b1b26d" xlink:to="loc_us-gaap_PerformanceSharesMember_019e6493-a8b2-7d54-858e-608cd138dc4c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_PerformanceSharesGranted2025AndPriorMember_019e6493-a8b2-7be5-b996-61b7492277d9" xlink:href="kop-20260630.xsd#kop_PerformanceSharesGranted2025AndPriorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PerformanceSharesMember_019e6493-a8b2-7d54-858e-608cd138dc4c" xlink:to="loc_kop_PerformanceSharesGranted2025AndPriorMember_019e6493-a8b2-7be5-b996-61b7492277d9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_PerformanceSharesGrantedIn2026Member_019e6493-a8b2-798a-a3f8-b203f1700cd4" xlink:href="kop-20260630.xsd#kop_PerformanceSharesGrantedIn2026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PerformanceSharesMember_019e6493-a8b2-7d54-858e-608cd138dc4c" xlink:to="loc_kop_PerformanceSharesGrantedIn2026Member_019e6493-a8b2-798a-a3f8-b203f1700cd4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_MarketConditionUnitsMember_019e6493-a8b2-7bf4-8ed1-8c3c3f1024c6" xlink:href="kop-20260630.xsd#kop_MarketConditionUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6493-a8b2-725d-b32a-b7bd13b1b26d" xlink:to="loc_kop_MarketConditionUnitsMember_019e6493-a8b2-7bf4-8ed1-8c3c3f1024c6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019e6493-a8b2-7883-992b-e1bdab0adfbc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6493-a8b2-7630-a125-cbf7a1ff59bc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019e6493-a8b2-7883-992b-e1bdab0adfbc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_019e6493-a8b2-77df-b713-ce16357c36a8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6493-a8b2-7630-a125-cbf7a1ff59bc" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_019e6493-a8b2-77df-b713-ce16357c36a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardCumulativeAdjustedEarningPerSharePercentageOfTargetAward_019e6493-a8b2-76a0-a2bf-b12c2d56df9d" xlink:href="kop-20260630.xsd#kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardCumulativeAdjustedEarningPerSharePercentageOfTargetAward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6493-a8b2-7630-a125-cbf7a1ff59bc" xlink:to="loc_kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardCumulativeAdjustedEarningPerSharePercentageOfTargetAward_019e6493-a8b2-76a0-a2bf-b12c2d56df9d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardCumulativeFreeCashFlowPercentageOfTargetAward_019e6493-a8b2-76ff-a8a6-8e3945b0d240" xlink:href="kop-20260630.xsd#kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardCumulativeFreeCashFlowPercentageOfTargetAward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6493-a8b2-7630-a125-cbf7a1ff59bc" xlink:to="loc_kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardCumulativeFreeCashFlowPercentageOfTargetAward_019e6493-a8b2-76ff-a8a6-8e3945b0d240" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardAdjustedEBITDAMarginPerformanceModifierPercentage_019e6493-a8b2-7cc9-a5c6-66b6e15559db" xlink:href="kop-20260630.xsd#kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardAdjustedEBITDAMarginPerformanceModifierPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6493-a8b2-7630-a125-cbf7a1ff59bc" xlink:to="loc_kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardAdjustedEBITDAMarginPerformanceModifierPercentage_019e6493-a8b2-7cc9-a5c6-66b6e15559db" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfTargetAwardParticipantsEarn_019e6493-a8b2-777a-8d06-8b858ad494cd" xlink:href="kop-20260630.xsd#kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfTargetAwardParticipantsEarn"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6493-a8b2-7630-a125-cbf7a1ff59bc" xlink:to="loc_kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfTargetAwardParticipantsEarn_019e6493-a8b2-777a-8d06-8b858ad494cd" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsToBeVestedIfPerformanceCriteriaNotAchieved_019e6493-a8b2-7b6c-b1c5-2845395e58a9" xlink:href="kop-20260630.xsd#kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsToBeVestedIfPerformanceCriteriaNotAchieved"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6493-a8b2-7630-a125-cbf7a1ff59bc" xlink:to="loc_kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsToBeVestedIfPerformanceCriteriaNotAchieved_019e6493-a8b2-7b6c-b1c5-2845395e58a9" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e6493-a8b2-7059-b5dc-6671c690e74b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6493-a8b2-7630-a125-cbf7a1ff59bc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e6493-a8b2-7059-b5dc-6671c690e74b" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.koppers.com/role/StockbasedCompensationSummaryofStockOptionsandPerformanceAwardFairValueAssumptionsDetails" xlink:type="simple" xlink:href="kop-20260630.xsd#StockbasedCompensationSummaryofStockOptionsandPerformanceAwardFairValueAssumptionsDetails"/>
  <link:definitionLink xlink:role="http://www.koppers.com/role/StockbasedCompensationSummaryofStockOptionsandPerformanceAwardFairValueAssumptionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6493-a8b2-7b25-9667-6d02a1bf5f76" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e6493-a8b2-7965-945f-01e659c6eca4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6493-a8b2-7b25-9667-6d02a1bf5f76" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e6493-a8b2-7965-945f-01e659c6eca4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateAxis_019e6493-a8b2-7149-9537-d3614c6d6405" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e6493-a8b2-7965-945f-01e659c6eca4" xlink:to="loc_us-gaap_AwardDateAxis_019e6493-a8b2-7149-9537-d3614c6d6405" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateDomain_019e6493-a8b2-7149-9537-d3614c6d6405_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardDateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardDateAxis_019e6493-a8b2-7149-9537-d3614c6d6405" xlink:to="loc_us-gaap_AwardDateDomain_019e6493-a8b2-7149-9537-d3614c6d6405_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateDomain_019e6493-a8b2-7a40-a9bc-2f0166e5bbea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardDateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardDateAxis_019e6493-a8b2-7149-9537-d3614c6d6405" xlink:to="loc_us-gaap_AwardDateDomain_019e6493-a8b2-7a40-a9bc-2f0166e5bbea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_JanuaryTwoThousandTwentySixGrantMember_019e6493-a8b2-74b6-8633-7371935da5b4" xlink:href="kop-20260630.xsd#kop_JanuaryTwoThousandTwentySixGrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_019e6493-a8b2-7a40-a9bc-2f0166e5bbea" xlink:to="loc_kop_JanuaryTwoThousandTwentySixGrantMember_019e6493-a8b2-74b6-8633-7371935da5b4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_MayTwoThousandTwentySixGrantMember_019fa386-26b7-7700-8d46-b30360f3a8dd" xlink:href="kop-20260630.xsd#kop_MayTwoThousandTwentySixGrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_019e6493-a8b2-7a40-a9bc-2f0166e5bbea" xlink:to="loc_kop_MayTwoThousandTwentySixGrantMember_019fa386-26b7-7700-8d46-b30360f3a8dd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e6493-a8b2-7ee3-bb40-2289e1c1c3e5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e6493-a8b2-7965-945f-01e659c6eca4" xlink:to="loc_us-gaap_AwardTypeAxis_019e6493-a8b2-7ee3-bb40-2289e1c1c3e5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6493-a8b2-7ee3-bb40-2289e1c1c3e5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e6493-a8b2-7ee3-bb40-2289e1c1c3e5" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6493-a8b2-7ee3-bb40-2289e1c1c3e5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6493-a8b2-7b23-872b-6ed89cb22b36" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e6493-a8b2-7ee3-bb40-2289e1c1c3e5" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6493-a8b2-7b23-872b-6ed89cb22b36" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019e6493-a8b2-74f8-a174-94a607bb8cff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6493-a8b2-7b23-872b-6ed89cb22b36" xlink:to="loc_us-gaap_PerformanceSharesMember_019e6493-a8b2-74f8-a174-94a607bb8cff" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExercisePrice_019e6493-a8b2-771a-8696-3a4600118cf5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6493-a8b2-7b25-9667-6d02a1bf5f76" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExercisePrice_019e6493-a8b2-771a-8696-3a4600118cf5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_019e6493-a8b2-70d8-8177-499a14ffcf2f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6493-a8b2-7b25-9667-6d02a1bf5f76" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_019e6493-a8b2-70d8-8177-499a14ffcf2f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_019e6493-a8b2-740d-9090-e783db405934" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6493-a8b2-7b25-9667-6d02a1bf5f76" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_019e6493-a8b2-740d-9090-e783db405934" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsLookBackPeriod_019e6493-a8b2-70e4-a9f7-733e465a64c5" xlink:href="kop-20260630.xsd#kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsLookBackPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6493-a8b2-7b25-9667-6d02a1bf5f76" xlink:to="loc_kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsLookBackPeriod_019e6493-a8b2-70e4-a9f7-733e465a64c5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e6493-a8b2-7283-b26a-64f90d45767b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6493-a8b2-7b25-9667-6d02a1bf5f76" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e6493-a8b2-7283-b26a-64f90d45767b" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.koppers.com/role/StockbasedCompensationSummaryofStatusandActivityofNonVestedStockUnitsDetails" xlink:type="simple" xlink:href="kop-20260630.xsd#StockbasedCompensationSummaryofStatusandActivityofNonVestedStockUnitsDetails"/>
  <link:definitionLink xlink:role="http://www.koppers.com/role/StockbasedCompensationSummaryofStatusandActivityofNonVestedStockUnitsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6493-a8b2-74fb-9ce3-1c24899d9577" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e6493-a8b2-77c0-b170-adb8506ee6d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6493-a8b2-74fb-9ce3-1c24899d9577" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e6493-a8b2-77c0-b170-adb8506ee6d2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e6493-a8b2-7f95-aa5e-d2d3b440e4da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e6493-a8b2-77c0-b170-adb8506ee6d2" xlink:to="loc_us-gaap_AwardTypeAxis_019e6493-a8b2-7f95-aa5e-d2d3b440e4da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6493-a8b2-7f95-aa5e-d2d3b440e4da_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e6493-a8b2-7f95-aa5e-d2d3b440e4da" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6493-a8b2-7f95-aa5e-d2d3b440e4da_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6493-a8b2-7aa8-879f-bd9b57778793" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e6493-a8b2-7f95-aa5e-d2d3b440e4da" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6493-a8b2-7aa8-879f-bd9b57778793" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e6493-a8b2-75ce-ae42-ca43638bbcaf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6493-a8b2-7aa8-879f-bd9b57778793" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e6493-a8b2-75ce-ae42-ca43638bbcaf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019e6493-a8b2-7707-abdd-d0f185dc59e7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e6493-a8b2-7aa8-879f-bd9b57778793" xlink:to="loc_us-gaap_PerformanceSharesMember_019e6493-a8b2-7707-abdd-d0f185dc59e7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e6493-a8b2-7be5-8275-18a9a8904d83" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6493-a8b2-74fb-9ce3-1c24899d9577" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e6493-a8b2-7be5-8275-18a9a8904d83" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e6493-a8b2-7d47-8a31-0cf47e88c3db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e6493-a8b2-7be5-8275-18a9a8904d83" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e6493-a8b2-7d47-8a31-0cf47e88c3db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e6493-a8b2-7e97-b4b9-d9c714213b5d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e6493-a8b2-7be5-8275-18a9a8904d83" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e6493-a8b2-7e97-b4b9-d9c714213b5d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_ShareBasedCompensationArrangementByCreditedFromDividends_019e6493-a8b2-75fc-b13b-89fd18f64960" xlink:href="kop-20260630.xsd#kop_ShareBasedCompensationArrangementByCreditedFromDividends"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e6493-a8b2-7be5-8275-18a9a8904d83" xlink:to="loc_kop_ShareBasedCompensationArrangementByCreditedFromDividends_019e6493-a8b2-75fc-b13b-89fd18f64960" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_ShareBasedCompensationArrangementByPerformanceShareAdjustment_019e6493-a8b2-7a69-94db-bfb8aeba6f36" xlink:href="kop-20260630.xsd#kop_ShareBasedCompensationArrangementByPerformanceShareAdjustment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e6493-a8b2-7be5-8275-18a9a8904d83" xlink:to="loc_kop_ShareBasedCompensationArrangementByPerformanceShareAdjustment_019e6493-a8b2-7a69-94db-bfb8aeba6f36" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019e6493-a8b2-7b2f-b860-5965b991e184" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e6493-a8b2-7be5-8275-18a9a8904d83" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019e6493-a8b2-7b2f-b860-5965b991e184" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019e6493-a8b2-71c4-8e4a-6ad3730ca931" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e6493-a8b2-7be5-8275-18a9a8904d83" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019e6493-a8b2-71c4-8e4a-6ad3730ca931" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e6493-a8b2-75d4-9c85-dd2385df00f6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e6493-a8b2-7c54-974a-3bfd37cf33a7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e6493-a8b2-74fb-9ce3-1c24899d9577" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e6493-a8b2-7c54-974a-3bfd37cf33a7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e6493-a8b2-729a-a52e-e20822d8a927" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e6493-a8b2-7c54-974a-3bfd37cf33a7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e6493-a8b2-729a-a52e-e20822d8a927" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e6493-a8b2-77b2-8f29-259bcc12a7f2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e6493-a8b2-7c54-974a-3bfd37cf33a7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e6493-a8b2-77b2-8f29-259bcc12a7f2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsCreditedFromDividends_019e6493-a8b2-7601-9d47-351462e34a4d" xlink:href="kop-20260630.xsd#kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsCreditedFromDividends"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e6493-a8b2-7c54-974a-3bfd37cf33a7" xlink:to="loc_kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsCreditedFromDividends_019e6493-a8b2-7601-9d47-351462e34a4d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformanceShareAdjustment_019e6493-a8b2-7f39-bb92-50179f592d9d" xlink:href="kop-20260630.xsd#kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformanceShareAdjustment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e6493-a8b2-7c54-974a-3bfd37cf33a7" xlink:to="loc_kop_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformanceShareAdjustment_019e6493-a8b2-7f39-bb92-50179f592d9d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019e6493-a8b2-76cb-a39b-522bafc7b174" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e6493-a8b2-7c54-974a-3bfd37cf33a7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019e6493-a8b2-76cb-a39b-522bafc7b174" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019e6493-a8b2-7ac0-af72-7b06d8738d5f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e6493-a8b2-7c54-974a-3bfd37cf33a7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019e6493-a8b2-7ac0-af72-7b06d8738d5f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e6493-a8b2-711a-a77e-f452c43eeb9b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.koppers.com/role/SegmentInformationSummaryofSegmentRevenuesforSignificantProductLinesDetails" xlink:type="simple" xlink:href="kop-20260630.xsd#SegmentInformationSummaryofSegmentRevenuesforSignificantProductLinesDetails"/>
  <link:definitionLink xlink:role="http://www.koppers.com/role/SegmentInformationSummaryofSegmentRevenuesforSignificantProductLinesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019e6493-a8b2-78dd-a08a-6cacac6b9d43" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e6493-a8b2-7464-9c68-c0fd7bd595ab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6493-a8b2-78dd-a08a-6cacac6b9d43" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e6493-a8b2-7464-9c68-c0fd7bd595ab" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e6493-a8b2-7a6d-b177-f4627da323dc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e6493-a8b2-7464-9c68-c0fd7bd595ab" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e6493-a8b2-7a6d-b177-f4627da323dc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e6493-a8b2-7a6d-b177-f4627da323dc_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e6493-a8b2-7a6d-b177-f4627da323dc" xlink:to="loc_us-gaap_SegmentDomain_019e6493-a8b2-7a6d-b177-f4627da323dc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e6493-a8b2-7793-ab80-91da95ef8225" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e6493-a8b2-7a6d-b177-f4627da323dc" xlink:to="loc_us-gaap_SegmentDomain_019e6493-a8b2-7793-ab80-91da95ef8225" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_RailroadAndUtilityProductsAndServicesMember_019e6493-a8b2-7bf3-b146-513fe891c765" xlink:href="kop-20260630.xsd#kop_RailroadAndUtilityProductsAndServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6493-a8b2-7793-ab80-91da95ef8225" xlink:to="loc_kop_RailroadAndUtilityProductsAndServicesMember_019e6493-a8b2-7bf3-b146-513fe891c765" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_PerformanceChemicalsMember_019e6493-a8b2-750e-b80a-533fcb0a2f73" xlink:href="kop-20260630.xsd#kop_PerformanceChemicalsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6493-a8b2-7793-ab80-91da95ef8225" xlink:to="loc_kop_PerformanceChemicalsMember_019e6493-a8b2-750e-b80a-533fcb0a2f73" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_CarbonMaterialsAndChemicalsMember_019e6493-a8b2-7758-a4ab-7ac0c98a64c5" xlink:href="kop-20260630.xsd#kop_CarbonMaterialsAndChemicalsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6493-a8b2-7793-ab80-91da95ef8225" xlink:to="loc_kop_CarbonMaterialsAndChemicalsMember_019e6493-a8b2-7758-a4ab-7ac0c98a64c5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019e6493-a8b2-70d4-8e00-b093115e8bba" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e6493-a8b2-7464-9c68-c0fd7bd595ab" xlink:to="loc_srt_ProductOrServiceAxis_019e6493-a8b2-70d4-8e00-b093115e8bba" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e6493-a8b2-70d4-8e00-b093115e8bba_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019e6493-a8b2-70d4-8e00-b093115e8bba" xlink:to="loc_srt_ProductsAndServicesDomain_019e6493-a8b2-70d4-8e00-b093115e8bba_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e6493-a8b2-7abf-8644-46c76a39a9c9" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019e6493-a8b2-70d4-8e00-b093115e8bba" xlink:to="loc_srt_ProductsAndServicesDomain_019e6493-a8b2-7abf-8644-46c76a39a9c9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_RailroadTreatedProductsMember_019e6493-a8b2-72fb-9900-8cd97e126e16" xlink:href="kop-20260630.xsd#kop_RailroadTreatedProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e6493-a8b2-7abf-8644-46c76a39a9c9" xlink:to="loc_kop_RailroadTreatedProductsMember_019e6493-a8b2-72fb-9900-8cd97e126e16" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_UtilityPolesMember_019e6493-a8b2-7361-b379-bcc9cb82a9a8" xlink:href="kop-20260630.xsd#kop_UtilityPolesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e6493-a8b2-7abf-8644-46c76a39a9c9" xlink:to="loc_kop_UtilityPolesMember_019e6493-a8b2-7361-b379-bcc9cb82a9a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_RailroadInfrastructureProductsAndServicesMember_019e6493-a8b2-7498-8c6b-4f6fc3113944" xlink:href="kop-20260630.xsd#kop_RailroadInfrastructureProductsAndServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e6493-a8b2-7abf-8644-46c76a39a9c9" xlink:to="loc_kop_RailroadInfrastructureProductsAndServicesMember_019e6493-a8b2-7498-8c6b-4f6fc3113944" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_WoodPreservativeProductsAndOtherMember_019e6493-a8b2-78ab-987c-1b80aacda911" xlink:href="kop-20260630.xsd#kop_WoodPreservativeProductsAndOtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e6493-a8b2-7abf-8644-46c76a39a9c9" xlink:to="loc_kop_WoodPreservativeProductsAndOtherMember_019e6493-a8b2-78ab-987c-1b80aacda911" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_PitchAndRelatedProductsMember_019e6493-a8b2-77ae-b262-2fe5235c2302" xlink:href="kop-20260630.xsd#kop_PitchAndRelatedProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e6493-a8b2-7abf-8644-46c76a39a9c9" xlink:to="loc_kop_PitchAndRelatedProductsMember_019e6493-a8b2-77ae-b262-2fe5235c2302" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_CarbonBlackFeedstockAndDistillatesMember_019e6493-a8b2-7c15-a8e8-cf88ba45563a" xlink:href="kop-20260630.xsd#kop_CarbonBlackFeedstockAndDistillatesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e6493-a8b2-7abf-8644-46c76a39a9c9" xlink:to="loc_kop_CarbonBlackFeedstockAndDistillatesMember_019e6493-a8b2-7c15-a8e8-cf88ba45563a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_NaphthalenePhthalicAnhydrideAndOtherChemicalsMember_019e6493-a8b2-7a86-ae5e-e4fc9544092e" xlink:href="kop-20260630.xsd#kop_NaphthalenePhthalicAnhydrideAndOtherChemicalsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e6493-a8b2-7abf-8644-46c76a39a9c9" xlink:to="loc_kop_NaphthalenePhthalicAnhydrideAndOtherChemicalsMember_019e6493-a8b2-7a86-ae5e-e4fc9544092e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_019e6493-a8b2-74c2-9c66-5d69de264fcc" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e6493-a8b2-7464-9c68-c0fd7bd595ab" xlink:to="loc_srt_ConsolidationItemsAxis_019e6493-a8b2-74c2-9c66-5d69de264fcc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019e6493-a8b2-74c2-9c66-5d69de264fcc_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_019e6493-a8b2-74c2-9c66-5d69de264fcc" xlink:to="loc_srt_ConsolidationItemsDomain_019e6493-a8b2-74c2-9c66-5d69de264fcc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019e6493-a8b2-736f-b27e-5621b3d72ef6" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_019e6493-a8b2-74c2-9c66-5d69de264fcc" xlink:to="loc_srt_ConsolidationItemsDomain_019e6493-a8b2-736f-b27e-5621b3d72ef6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_019e6493-a8b2-7c07-b37b-94fb9a7d5784" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019e6493-a8b2-736f-b27e-5621b3d72ef6" xlink:to="loc_us-gaap_OperatingSegmentsMember_019e6493-a8b2-7c07-b37b-94fb9a7d5784" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e6493-a8b2-7f8e-b39a-defdb2085fcd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6493-a8b2-78dd-a08a-6cacac6b9d43" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e6493-a8b2-7f8e-b39a-defdb2085fcd" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.koppers.com/role/SegmentInformationSummaryofResultsofSegmentOperationsDetails" xlink:type="simple" xlink:href="kop-20260630.xsd#SegmentInformationSummaryofResultsofSegmentOperationsDetails"/>
  <link:definitionLink xlink:role="http://www.koppers.com/role/SegmentInformationSummaryofResultsofSegmentOperationsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e6493-a8b2-7f32-a2ab-a2087f404dbd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_019e6493-a8b2-7da1-bd27-fc38d4200d1d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e6493-a8b2-7f32-a2ab-a2087f404dbd" xlink:to="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_019e6493-a8b2-7da1-bd27-fc38d4200d1d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e6493-a8b2-7617-b009-5b9e076caa11" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_019e6493-a8b2-7da1-bd27-fc38d4200d1d" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e6493-a8b2-7617-b009-5b9e076caa11" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e6493-a8b2-7617-b009-5b9e076caa11_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e6493-a8b2-7617-b009-5b9e076caa11" xlink:to="loc_us-gaap_SegmentDomain_019e6493-a8b2-7617-b009-5b9e076caa11_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e6493-a8b2-77c3-8d54-0b902656e305" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e6493-a8b2-7617-b009-5b9e076caa11" xlink:to="loc_us-gaap_SegmentDomain_019e6493-a8b2-77c3-8d54-0b902656e305" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_RailroadAndUtilityProductsAndServicesMember_019e6493-a8b2-77a8-93f5-c5976c842355" xlink:href="kop-20260630.xsd#kop_RailroadAndUtilityProductsAndServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6493-a8b2-77c3-8d54-0b902656e305" xlink:to="loc_kop_RailroadAndUtilityProductsAndServicesMember_019e6493-a8b2-77a8-93f5-c5976c842355" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_PerformanceChemicalsMember_019e6493-a8b2-76ef-ba05-fb25323913bc" xlink:href="kop-20260630.xsd#kop_PerformanceChemicalsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6493-a8b2-77c3-8d54-0b902656e305" xlink:to="loc_kop_PerformanceChemicalsMember_019e6493-a8b2-76ef-ba05-fb25323913bc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_CarbonMaterialsAndChemicalsMember_019e6493-a8b2-72cc-a2c6-1cb2ef11b1a8" xlink:href="kop-20260630.xsd#kop_CarbonMaterialsAndChemicalsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6493-a8b2-77c3-8d54-0b902656e305" xlink:to="loc_kop_CarbonMaterialsAndChemicalsMember_019e6493-a8b2-72cc-a2c6-1cb2ef11b1a8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_019e6493-a8b2-75b8-a229-54a5d3f8a4ab" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_019e6493-a8b2-7da1-bd27-fc38d4200d1d" xlink:to="loc_srt_ConsolidationItemsAxis_019e6493-a8b2-75b8-a229-54a5d3f8a4ab" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019e6493-a8b2-75b8-a229-54a5d3f8a4ab_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_019e6493-a8b2-75b8-a229-54a5d3f8a4ab" xlink:to="loc_srt_ConsolidationItemsDomain_019e6493-a8b2-75b8-a229-54a5d3f8a4ab_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019e6493-a8b2-7f9d-aeb4-7c7a0c31f5ae" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_019e6493-a8b2-75b8-a229-54a5d3f8a4ab" xlink:to="loc_srt_ConsolidationItemsDomain_019e6493-a8b2-7f9d-aeb4-7c7a0c31f5ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_019e6493-a8b2-7b8e-b1c0-2a8611afe72d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019e6493-a8b2-7f9d-aeb4-7c7a0c31f5ae" xlink:to="loc_us-gaap_OperatingSegmentsMember_019e6493-a8b2-7b8e-b1c0-2a8611afe72d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntersegmentEliminationMember_019e6493-a8b2-7532-a889-8ebbbfbbc4b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IntersegmentEliminationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019e6493-a8b2-7f9d-aeb4-7c7a0c31f5ae" xlink:to="loc_us-gaap_IntersegmentEliminationMember_019e6493-a8b2-7532-a889-8ebbbfbbc4b3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateNonSegmentMember_019e6493-a8b2-768a-8934-67a9cb29f30a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateNonSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019e6493-a8b2-7f9d-aeb4-7c7a0c31f5ae" xlink:to="loc_us-gaap_CorporateNonSegmentMember_019e6493-a8b2-768a-8934-67a9cb29f30a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServicesSold_019e6493-a8b2-7e2f-9109-0ef70e3711b7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CostOfGoodsAndServicesSold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e6493-a8b2-7f32-a2ab-a2087f404dbd" xlink:to="loc_us-gaap_CostOfGoodsAndServicesSold_019e6493-a8b2-7e2f-9109-0ef70e3711b7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpense_019e6493-a8b2-70e8-9f00-a098bc8a504b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SellingGeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e6493-a8b2-7f32-a2ab-a2087f404dbd" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpense_019e6493-a8b2-70e8-9f00-a098bc8a504b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_OtherIncomeExpenseToReconcileToAdjustedEarningsBeforeInterestTaxDepreciationAndAmortization_019e6493-a8b2-739a-ba36-ccadd6d91526" xlink:href="kop-20260630.xsd#kop_OtherIncomeExpenseToReconcileToAdjustedEarningsBeforeInterestTaxDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e6493-a8b2-7f32-a2ab-a2087f404dbd" xlink:to="loc_kop_OtherIncomeExpenseToReconcileToAdjustedEarningsBeforeInterestTaxDepreciationAndAmortization_019e6493-a8b2-739a-ba36-ccadd6d91526" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_AdjustmentsToEarningsBeforeInterestTaxesDepreciationAndAmortizationAbstract_019e6493-a8b2-7a24-a75f-eafdb15b203f" xlink:href="kop-20260630.xsd#kop_AdjustmentsToEarningsBeforeInterestTaxesDepreciationAndAmortizationAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e6493-a8b2-7f32-a2ab-a2087f404dbd" xlink:to="loc_kop_AdjustmentsToEarningsBeforeInterestTaxesDepreciationAndAmortizationAbstract_019e6493-a8b2-7a24-a75f-eafdb15b203f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_AdjustedEarningsBeforeInterestTaxDepreciationAndAmortization_019e6493-a8b2-7390-9db9-9d439f70bbb3" xlink:href="kop-20260630.xsd#kop_AdjustedEarningsBeforeInterestTaxDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_kop_AdjustmentsToEarningsBeforeInterestTaxesDepreciationAndAmortizationAbstract_019e6493-a8b2-7a24-a75f-eafdb15b203f" xlink:to="loc_kop_AdjustedEarningsBeforeInterestTaxDepreciationAndAmortization_019e6493-a8b2-7390-9db9-9d439f70bbb3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_ItemsExcludedFromDeterminationOfSegmentProfitAbstract_019e6493-a8b2-7c94-bd99-c75c120d8999" xlink:href="kop-20260630.xsd#kop_ItemsExcludedFromDeterminationOfSegmentProfitAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e6493-a8b2-7f32-a2ab-a2087f404dbd" xlink:to="loc_kop_ItemsExcludedFromDeterminationOfSegmentProfitAbstract_019e6493-a8b2-7c94-bd99-c75c120d8999" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_AcquisitionInventoryStepUpAmortization_019e6493-a8b2-7278-85cf-b1f541d4f7e1" xlink:href="kop-20260630.xsd#kop_AcquisitionInventoryStepUpAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_kop_ItemsExcludedFromDeterminationOfSegmentProfitAbstract_019e6493-a8b2-7c94-bd99-c75c120d8999" xlink:to="loc_kop_AcquisitionInventoryStepUpAmortization_019e6493-a8b2-7278-85cf-b1f541d4f7e1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedComputerSoftwareAmortization1_019e6493-a8b2-70fd-b668-128f902b1b99" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CapitalizedComputerSoftwareAmortization1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_kop_ItemsExcludedFromDeterminationOfSegmentProfitAbstract_019e6493-a8b2-7c94-bd99-c75c120d8999" xlink:to="loc_us-gaap_CapitalizedComputerSoftwareAmortization1_019e6493-a8b2-70fd-b668-128f902b1b99" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnDispositionOfAssets1_019e6493-a8b2-74fc-991f-b83b0dd8758e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GainLossOnDispositionOfAssets1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_kop_ItemsExcludedFromDeterminationOfSegmentProfitAbstract_019e6493-a8b2-7c94-bd99-c75c120d8999" xlink:to="loc_us-gaap_GainLossOnDispositionOfAssets1_019e6493-a8b2-74fc-991f-b83b0dd8758e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_ImpairmentRestructuringAndPlantClosureBenefitsCosts_019e6493-a8b2-7296-835d-9b8b0e431ec9" xlink:href="kop-20260630.xsd#kop_ImpairmentRestructuringAndPlantClosureBenefitsCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_kop_ItemsExcludedFromDeterminationOfSegmentProfitAbstract_019e6493-a8b2-7c94-bd99-c75c120d8999" xlink:to="loc_kop_ImpairmentRestructuringAndPlantClosureBenefitsCosts_019e6493-a8b2-7296-835d-9b8b0e431ec9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryLIFOReserveEffectOnIncomeNet_019e6493-a8b2-7f34-bc1a-b2c6c6223fde" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InventoryLIFOReserveEffectOnIncomeNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_kop_ItemsExcludedFromDeterminationOfSegmentProfitAbstract_019e6493-a8b2-7c94-bd99-c75c120d8999" xlink:to="loc_us-gaap_InventoryLIFOReserveEffectOnIncomeNet_019e6493-a8b2-7f34-bc1a-b2c6c6223fde" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_ChangesInDerivativeLiability_019e6493-a8b2-757b-9453-eb727db0e98e" xlink:href="kop-20260630.xsd#kop_ChangesInDerivativeLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_kop_ItemsExcludedFromDeterminationOfSegmentProfitAbstract_019e6493-a8b2-7c94-bd99-c75c120d8999" xlink:to="loc_kop_ChangesInDerivativeLiability_019e6493-a8b2-757b-9453-eb727db0e98e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_DefinedBenefitPlanGainLossDueToSettlementAndExpense_019e6493-a8b2-70a3-9997-81fbaae6505f" xlink:href="kop-20260630.xsd#kop_DefinedBenefitPlanGainLossDueToSettlementAndExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_kop_ItemsExcludedFromDeterminationOfSegmentProfitAbstract_019e6493-a8b2-7c94-bd99-c75c120d8999" xlink:to="loc_kop_DefinedBenefitPlanGainLossDueToSettlementAndExpense_019e6493-a8b2-70a3-9997-81fbaae6505f" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_DepreciationAndAmortizationIncludingImpairmentAndRestructuring_019e6493-a8b2-71e5-bc03-681197b30f27" xlink:href="kop-20260630.xsd#kop_DepreciationAndAmortizationIncludingImpairmentAndRestructuring"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_kop_ItemsExcludedFromDeterminationOfSegmentProfitAbstract_019e6493-a8b2-7c94-bd99-c75c120d8999" xlink:to="loc_kop_DepreciationAndAmortizationIncludingImpairmentAndRestructuring_019e6493-a8b2-71e5-bc03-681197b30f27" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_InterestExpenseExcludedFromDeterminationOfSegmentProfit_019e6493-a8b2-75a5-9280-6607aeb6e4eb" xlink:href="kop-20260630.xsd#kop_InterestExpenseExcludedFromDeterminationOfSegmentProfit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_kop_ItemsExcludedFromDeterminationOfSegmentProfitAbstract_019e6493-a8b2-7c94-bd99-c75c120d8999" xlink:to="loc_kop_InterestExpenseExcludedFromDeterminationOfSegmentProfit_019e6493-a8b2-75a5-9280-6607aeb6e4eb" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_019e6493-a8b2-7c18-b1f1-bff9e02be62f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_kop_ItemsExcludedFromDeterminationOfSegmentProfitAbstract_019e6493-a8b2-7c94-bd99-c75c120d8999" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_019e6493-a8b2-7c18-b1f1-bff9e02be62f" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_019e6493-a8b2-7cdc-aae6-5704229a55fa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_kop_ItemsExcludedFromDeterminationOfSegmentProfitAbstract_019e6493-a8b2-7c94-bd99-c75c120d8999" xlink:to="loc_us-gaap_ProfitLoss_019e6493-a8b2-7cdc-aae6-5704229a55fa" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e6493-a8b2-7436-aafb-986a5e8c8690" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e6493-a8b2-7f32-a2ab-a2087f404dbd" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e6493-a8b2-7436-aafb-986a5e8c8690" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationAndAmortization_019e6493-a8b2-74b8-95e8-bb7cff92b3ad" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e6493-a8b2-7f32-a2ab-a2087f404dbd" xlink:to="loc_us-gaap_DepreciationAndAmortization_019e6493-a8b2-74b8-95e8-bb7cff92b3ad" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentExpenditureAdditionToLongLivedAssets_019e6493-a8b2-71a0-a6ba-f2192d8dd663" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentExpenditureAdditionToLongLivedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e6493-a8b2-7f32-a2ab-a2087f404dbd" xlink:to="loc_us-gaap_SegmentExpenditureAdditionToLongLivedAssets_019e6493-a8b2-71a0-a6ba-f2192d8dd663" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.koppers.com/role/SegmentInformationSummaryofTangibleandIntangibleAssetsbySegmentsDetails" xlink:type="simple" xlink:href="kop-20260630.xsd#SegmentInformationSummaryofTangibleandIntangibleAssetsbySegmentsDetails"/>
  <link:definitionLink xlink:role="http://www.koppers.com/role/SegmentInformationSummaryofTangibleandIntangibleAssetsbySegmentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019e6493-a8b2-7bd9-97d1-09c556c5694e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e6493-a8b2-72ef-ad7b-73576acaa454" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6493-a8b2-7bd9-97d1-09c556c5694e" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e6493-a8b2-72ef-ad7b-73576acaa454" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e6493-a8b2-7801-97ca-0226df874e58" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e6493-a8b2-72ef-ad7b-73576acaa454" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e6493-a8b2-7801-97ca-0226df874e58" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e6493-a8b2-7801-97ca-0226df874e58_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e6493-a8b2-7801-97ca-0226df874e58" xlink:to="loc_us-gaap_SegmentDomain_019e6493-a8b2-7801-97ca-0226df874e58_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e6493-a8b2-7600-a997-b41ca108fa74" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e6493-a8b2-7801-97ca-0226df874e58" xlink:to="loc_us-gaap_SegmentDomain_019e6493-a8b2-7600-a997-b41ca108fa74" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_RailroadAndUtilityProductsAndServicesMember_019e6493-a8b2-7c68-bf2c-30ba30c3dfb8" xlink:href="kop-20260630.xsd#kop_RailroadAndUtilityProductsAndServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6493-a8b2-7600-a997-b41ca108fa74" xlink:to="loc_kop_RailroadAndUtilityProductsAndServicesMember_019e6493-a8b2-7c68-bf2c-30ba30c3dfb8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_PerformanceChemicalsMember_019e6493-a8b2-7a00-a2b1-ce6b80e9dff0" xlink:href="kop-20260630.xsd#kop_PerformanceChemicalsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6493-a8b2-7600-a997-b41ca108fa74" xlink:to="loc_kop_PerformanceChemicalsMember_019e6493-a8b2-7a00-a2b1-ce6b80e9dff0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_CarbonMaterialsAndChemicalsMember_019e6493-a8b2-73c8-b30b-82c76a9fe796" xlink:href="kop-20260630.xsd#kop_CarbonMaterialsAndChemicalsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6493-a8b2-7600-a997-b41ca108fa74" xlink:to="loc_kop_CarbonMaterialsAndChemicalsMember_019e6493-a8b2-73c8-b30b-82c76a9fe796" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_019e6493-a8b2-7b94-87e2-579bbd431304" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e6493-a8b2-72ef-ad7b-73576acaa454" xlink:to="loc_srt_ConsolidationItemsAxis_019e6493-a8b2-7b94-87e2-579bbd431304" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019e6493-a8b2-7b94-87e2-579bbd431304_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_019e6493-a8b2-7b94-87e2-579bbd431304" xlink:to="loc_srt_ConsolidationItemsDomain_019e6493-a8b2-7b94-87e2-579bbd431304_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019e6493-a8b2-7e15-b8d8-f500fabed887" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_019e6493-a8b2-7b94-87e2-579bbd431304" xlink:to="loc_srt_ConsolidationItemsDomain_019e6493-a8b2-7e15-b8d8-f500fabed887" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_019e6493-a8b2-72e8-93e8-88893610e39e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019e6493-a8b2-7e15-b8d8-f500fabed887" xlink:to="loc_us-gaap_OperatingSegmentsMember_019e6493-a8b2-72e8-93e8-88893610e39e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateNonSegmentMember_019e6493-a8b2-7740-a305-ae10d6310657" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateNonSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019e6493-a8b2-7e15-b8d8-f500fabed887" xlink:to="loc_us-gaap_CorporateNonSegmentMember_019e6493-a8b2-7740-a305-ae10d6310657" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_019e6493-a8b2-76cb-a739-8aa7871a496e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6493-a8b2-7bd9-97d1-09c556c5694e" xlink:to="loc_us-gaap_Assets_019e6493-a8b2-76cb-a739-8aa7871a496e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_019e6493-a8b2-7eba-b6b4-abab5f3bf3e4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e6493-a8b2-7bd9-97d1-09c556c5694e" xlink:to="loc_us-gaap_Goodwill_019e6493-a8b2-7eba-b6b4-abab5f3bf3e4" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.koppers.com/role/IncomeTaxesAdditionalInformationDetails" xlink:type="simple" xlink:href="kop-20260630.xsd#IncomeTaxesAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.koppers.com/role/IncomeTaxesAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationLineItems_019fa332-f033-7dcc-bfc2-4499016bb463" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationTable_019fa332-f033-724d-ba06-cbbced705079" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateReconciliationLineItems_019fa332-f033-7dcc-bfc2-4499016bb463" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationTable_019fa332-f033-724d-ba06-cbbced705079" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanAxis_019fa332-f033-7184-a1f2-06fbf9d35ebf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringPlanAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateReconciliationTable_019fa332-f033-724d-ba06-cbbced705079" xlink:to="loc_us-gaap_RestructuringPlanAxis_019fa332-f033-7184-a1f2-06fbf9d35ebf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_019fa332-f033-7184-a1f2-06fbf9d35ebf_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringPlanAxis_019fa332-f033-7184-a1f2-06fbf9d35ebf" xlink:to="loc_us-gaap_RestructuringPlanDomain_019fa332-f033-7184-a1f2-06fbf9d35ebf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_019fa332-f033-7589-b7fd-a1dfe6c323dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringPlanAxis_019fa332-f033-7184-a1f2-06fbf9d35ebf" xlink:to="loc_us-gaap_RestructuringPlanDomain_019fa332-f033-7589-b7fd-a1dfe6c323dd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_StickneyShutdownMember_019fa332-f033-7adb-8fd4-e55812503082" xlink:href="kop-20260630.xsd#kop_StickneyShutdownMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_019fa332-f033-7589-b7fd-a1dfe6c323dd" xlink:to="loc_kop_StickneyShutdownMember_019fa332-f033-7adb-8fd4-e55812503082" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_019fa332-f033-7c38-8f44-5119b3c9b016" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EffectiveIncomeTaxRateContinuingOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateReconciliationLineItems_019fa332-f033-7dcc-bfc2-4499016bb463" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_019fa332-f033-7c38-8f44-5119b3c9b016" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_IncomeTaxExpensePercentageOfPreTaxIncomeLoss_019fa332-f033-77e6-a370-247b7e0a4c10" xlink:href="kop-20260630.xsd#kop_IncomeTaxExpensePercentageOfPreTaxIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateReconciliationLineItems_019fa332-f033-7dcc-bfc2-4499016bb463" xlink:to="loc_kop_IncomeTaxExpensePercentageOfPreTaxIncomeLoss_019fa332-f033-77e6-a370-247b7e0a4c10" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate_019fa332-f033-7f44-86de-7cd3c7918a5d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateReconciliationLineItems_019fa332-f033-7dcc-bfc2-4499016bb463" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate_019fa332-f033-7f44-86de-7cd3c7918a5d" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.koppers.com/role/PensionsandPostRetirementBenefitPlansAdditionalInformationDetails" xlink:type="simple" xlink:href="kop-20260630.xsd#PensionsandPostRetirementBenefitPlansAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.koppers.com/role/PensionsandPostRetirementBenefitPlansAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019e6493-a8b2-72ff-9493-bbbebc647dcb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_019e6493-a8b2-7949-bae4-4cfc6017912c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019e6493-a8b2-72ff-9493-bbbebc647dcb" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_019e6493-a8b2-7949-bae4-4cfc6017912c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_019e6493-a8b2-7c71-8690-2f4c65f73175" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetirementPlanTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_019e6493-a8b2-7949-bae4-4cfc6017912c" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_019e6493-a8b2-7c71-8690-2f4c65f73175" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_019e6493-a8b2-7c71-8690-2f4c65f73175_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetirementPlanTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_019e6493-a8b2-7c71-8690-2f4c65f73175" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_019e6493-a8b2-7c71-8690-2f4c65f73175_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_019e6493-a8b2-7f1c-a133-6f08affe8349" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetirementPlanTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_019e6493-a8b2-7c71-8690-2f4c65f73175" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_019e6493-a8b2-7f1c-a133-6f08affe8349" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_USQualifiedPensionPlanMember_019e6493-a8b2-7c11-8ba4-fe4902e878b2" xlink:href="kop-20260630.xsd#kop_USQualifiedPensionPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_019e6493-a8b2-7f1c-a133-6f08affe8349" xlink:to="loc_kop_USQualifiedPensionPlanMember_019e6493-a8b2-7c11-8ba4-fe4902e878b2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_019e6493-a8b2-75fd-a505-de252bada515" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_019e6493-a8b2-7949-bae4-4cfc6017912c" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_019e6493-a8b2-75fd-a505-de252bada515" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_019e6493-a8b2-75fd-a505-de252bada515_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_019e6493-a8b2-75fd-a505-de252bada515" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_019e6493-a8b2-75fd-a505-de252bada515_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_019e6493-a8b2-76fb-ada6-e12915d20413" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_019e6493-a8b2-75fd-a505-de252bada515" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_019e6493-a8b2-76fb-ada6-e12915d20413" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignPlanMember_019e6493-a8b2-739d-9b10-0c344c0e75f6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_019e6493-a8b2-76fb-ada6-e12915d20413" xlink:to="loc_us-gaap_ForeignPlanMember_019e6493-a8b2-739d-9b10-0c344c0e75f6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPensionPlanLiabilitiesNoncurrent_019e6493-a8b2-75af-8323-6174e0b17d6f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DefinedBenefitPensionPlanLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019e6493-a8b2-72ff-9493-bbbebc647dcb" xlink:to="loc_us-gaap_DefinedBenefitPensionPlanLiabilitiesNoncurrent_019e6493-a8b2-75af-8323-6174e0b17d6f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_AdditionalCashFunding_019e6493-a8b2-7248-b5db-106fdb2381aa" xlink:href="kop-20260630.xsd#kop_AdditionalCashFunding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019e6493-a8b2-72ff-9493-bbbebc647dcb" xlink:to="loc_kop_AdditionalCashFunding_019e6493-a8b2-7248-b5db-106fdb2381aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossRelatedToLitigationSettlement_019e6493-a8b2-7f87-a650-c17e2a411c9d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GainLossRelatedToLitigationSettlement"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019e6493-a8b2-72ff-9493-bbbebc647dcb" xlink:to="loc_us-gaap_GainLossRelatedToLitigationSettlement_019e6493-a8b2-7f87-a650-c17e2a411c9d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForProceedsFromLifeInsurancePolicies_019e6493-a8b2-7618-8ee8-f9ddf66dfde4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsForProceedsFromLifeInsurancePolicies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019e6493-a8b2-72ff-9493-bbbebc647dcb" xlink:to="loc_us-gaap_PaymentsForProceedsFromLifeInsurancePolicies_019e6493-a8b2-7618-8ee8-f9ddf66dfde4" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.koppers.com/role/DebtSummaryofLongTermDebtInstrumentsDetails" xlink:type="simple" xlink:href="kop-20260630.xsd#DebtSummaryofLongTermDebtInstrumentsDetails"/>
  <link:definitionLink xlink:role="http://www.koppers.com/role/DebtSummaryofLongTermDebtInstrumentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e6493-a8b2-7989-bf1e-8c8cdf70bfe3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e6493-a8b2-706f-9653-16081f653536" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6493-a8b2-7989-bf1e-8c8cdf70bfe3" xlink:to="loc_us-gaap_DebtInstrumentTable_019e6493-a8b2-706f-9653-16081f653536" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e6493-a8b2-7d70-b4c9-40608171ff01" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e6493-a8b2-706f-9653-16081f653536" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e6493-a8b2-7d70-b4c9-40608171ff01" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e6493-a8b2-7d70-b4c9-40608171ff01_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e6493-a8b2-7d70-b4c9-40608171ff01" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e6493-a8b2-7d70-b4c9-40608171ff01_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e6493-a8b2-74d8-984c-fb4f45e43d61" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e6493-a8b2-7d70-b4c9-40608171ff01" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e6493-a8b2-74d8-984c-fb4f45e43d61" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_SixPointOneNinePercentCreditFacilityMember_019e6493-a8b2-7dc2-8345-2db0e3bef74f" xlink:href="kop-20260630.xsd#kop_SixPointOneNinePercentCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e6493-a8b2-74d8-984c-fb4f45e43d61" xlink:to="loc_kop_SixPointOneNinePercentCreditFacilityMember_019e6493-a8b2-7dc2-8345-2db0e3bef74f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e6493-a8b2-72ac-b8c3-0c6f8f986b0e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e6493-a8b2-706f-9653-16081f653536" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e6493-a8b2-72ac-b8c3-0c6f8f986b0e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e6493-a8b2-72ac-b8c3-0c6f8f986b0e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e6493-a8b2-72ac-b8c3-0c6f8f986b0e" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e6493-a8b2-72ac-b8c3-0c6f8f986b0e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e6493-a8b2-70b5-a4b7-7aec72b54b6a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e6493-a8b2-72ac-b8c3-0c6f8f986b0e" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e6493-a8b2-70b5-a4b7-7aec72b54b6a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_CreditFacilityMember_019e6493-a8b2-79fa-bea1-ec04b735c35a" xlink:href="kop-20260630.xsd#kop_CreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e6493-a8b2-70b5-a4b7-7aec72b54b6a" xlink:to="loc_kop_CreditFacilityMember_019e6493-a8b2-79fa-bea1-ec04b735c35a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_TermLoanBMember_019e6493-a8b2-709f-a5fe-63941bf4261b" xlink:href="kop-20260630.xsd#kop_TermLoanBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e6493-a8b2-70b5-a4b7-7aec72b54b6a" xlink:to="loc_kop_TermLoanBMember_019e6493-a8b2-709f-a5fe-63941bf4261b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtWeightedAverageInterestRate_019e6493-a8b2-7f61-a639-e26796a4627a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtWeightedAverageInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6493-a8b2-7989-bf1e-8c8cdf70bfe3" xlink:to="loc_us-gaap_DebtWeightedAverageInterestRate_019e6493-a8b2-7f61-a639-e26796a4627a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_019e6493-a8b2-71bf-8ddb-de749f518b97" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6493-a8b2-7989-bf1e-8c8cdf70bfe3" xlink:to="loc_us-gaap_LongTermDebt_019e6493-a8b2-71bf-8ddb-de749f518b97" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtCurrent_019e6493-a8b2-7ce7-abe6-b1456328de70" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6493-a8b2-7989-bf1e-8c8cdf70bfe3" xlink:to="loc_us-gaap_LongTermDebtCurrent_019e6493-a8b2-7ce7-abe6-b1456328de70" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_019e6493-a8b2-74a3-b0cb-bff0752efa82" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6493-a8b2-7989-bf1e-8c8cdf70bfe3" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_019e6493-a8b2-74a3-b0cb-bff0752efa82" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_019e6493-a8b2-72a3-aa35-1fcb4ff225c8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6493-a8b2-7989-bf1e-8c8cdf70bfe3" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_019e6493-a8b2-72a3-aa35-1fcb4ff225c8" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.koppers.com/role/DebtAdditionalInformationDetails" xlink:type="simple" xlink:href="kop-20260630.xsd#DebtAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.koppers.com/role/DebtAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e6493-a8b2-7e59-af40-29f9f82e7ba4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e6493-a8b2-77db-81ec-19c656ae490b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6493-a8b2-7e59-af40-29f9f82e7ba4" xlink:to="loc_us-gaap_DebtInstrumentTable_019e6493-a8b2-77db-81ec-19c656ae490b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019e6493-a8b2-7dcd-9d10-3f1cd332cd1b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e6493-a8b2-77db-81ec-19c656ae490b" xlink:to="loc_us-gaap_CreditFacilityAxis_019e6493-a8b2-7dcd-9d10-3f1cd332cd1b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019e6493-a8b2-7dcd-9d10-3f1cd332cd1b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019e6493-a8b2-7dcd-9d10-3f1cd332cd1b" xlink:to="loc_us-gaap_CreditFacilityDomain_019e6493-a8b2-7dcd-9d10-3f1cd332cd1b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019e6493-a8b2-7905-a8fd-632be9ee2da3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019e6493-a8b2-7dcd-9d10-3f1cd332cd1b" xlink:to="loc_us-gaap_CreditFacilityDomain_019e6493-a8b2-7905-a8fd-632be9ee2da3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_RevolvingCreditAgreementMember_019e6493-a8b2-786f-8c07-650d95cf4bb4" xlink:href="kop-20260630.xsd#kop_RevolvingCreditAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019e6493-a8b2-7905-a8fd-632be9ee2da3" xlink:to="loc_kop_RevolvingCreditAgreementMember_019e6493-a8b2-786f-8c07-650d95cf4bb4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_SwinglineRevolvingCreditFacilityMember_019e6493-a8b2-7e76-9914-37669e8d13ab" xlink:href="kop-20260630.xsd#kop_SwinglineRevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019e6493-a8b2-7905-a8fd-632be9ee2da3" xlink:to="loc_kop_SwinglineRevolvingCreditFacilityMember_019e6493-a8b2-7e76-9914-37669e8d13ab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_CreditFacilityMember_019e6493-a8b2-7507-99dd-5c97aa2f9644" xlink:href="kop-20260630.xsd#kop_CreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019e6493-a8b2-7905-a8fd-632be9ee2da3" xlink:to="loc_kop_CreditFacilityMember_019e6493-a8b2-7507-99dd-5c97aa2f9644" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_AmendmentNumberOneToCreditFacilityMember_019e6493-a8b2-7a9b-a83c-2f8b0fe40b13" xlink:href="kop-20260630.xsd#kop_AmendmentNumberOneToCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019e6493-a8b2-7905-a8fd-632be9ee2da3" xlink:to="loc_kop_AmendmentNumberOneToCreditFacilityMember_019e6493-a8b2-7a9b-a83c-2f8b0fe40b13" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_AmendmentNumberTwoToCreditFacilityMember_019e6493-a8b2-74ba-afa1-63cccadf451d" xlink:href="kop-20260630.xsd#kop_AmendmentNumberTwoToCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019e6493-a8b2-7905-a8fd-632be9ee2da3" xlink:to="loc_kop_AmendmentNumberTwoToCreditFacilityMember_019e6493-a8b2-74ba-afa1-63cccadf451d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e6493-a8b2-74de-aa98-bbfb7455190e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e6493-a8b2-77db-81ec-19c656ae490b" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e6493-a8b2-74de-aa98-bbfb7455190e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e6493-a8b2-74de-aa98-bbfb7455190e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e6493-a8b2-74de-aa98-bbfb7455190e" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e6493-a8b2-74de-aa98-bbfb7455190e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e6493-a8b2-767f-bdba-2c8e4daaa8f3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e6493-a8b2-74de-aa98-bbfb7455190e" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e6493-a8b2-767f-bdba-2c8e4daaa8f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_SeniorSecuredCreditFacilitiesMember_019e6493-a8b2-7330-bf65-fdc515693a71" xlink:href="kop-20260630.xsd#kop_SeniorSecuredCreditFacilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e6493-a8b2-767f-bdba-2c8e4daaa8f3" xlink:to="loc_kop_SeniorSecuredCreditFacilitiesMember_019e6493-a8b2-7330-bf65-fdc515693a71" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e6493-a8b2-721e-9cd7-245927a63dcd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e6493-a8b2-77db-81ec-19c656ae490b" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e6493-a8b2-721e-9cd7-245927a63dcd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e6493-a8b2-721e-9cd7-245927a63dcd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e6493-a8b2-721e-9cd7-245927a63dcd" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e6493-a8b2-721e-9cd7-245927a63dcd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e6493-a8b2-7468-873b-18c521749e49" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e6493-a8b2-721e-9cd7-245927a63dcd" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e6493-a8b2-7468-873b-18c521749e49" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_TermLoanBMember_019e6493-a8b2-7016-9230-6142d143143e" xlink:href="kop-20260630.xsd#kop_TermLoanBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e6493-a8b2-7468-873b-18c521749e49" xlink:to="loc_kop_TermLoanBMember_019e6493-a8b2-7016-9230-6142d143143e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019e6493-a8b2-7294-8b4e-27421153a569" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6493-a8b2-7e59-af40-29f9f82e7ba4" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019e6493-a8b2-7294-8b4e-27421153a569" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnusedBorrowingCapacityAmount_019e6493-a8b2-7d4b-8f80-b68437d68b98" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentUnusedBorrowingCapacityAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6493-a8b2-7e59-af40-29f9f82e7ba4" xlink:to="loc_us-gaap_DebtInstrumentUnusedBorrowingCapacityAmount_019e6493-a8b2-7d4b-8f80-b68437d68b98" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_019e6493-a8b2-78cc-96e2-84d6586320c9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LettersOfCreditOutstandingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6493-a8b2-7e59-af40-29f9f82e7ba4" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_019e6493-a8b2-78cc-96e2-84d6586320c9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfSecuredDebt_019e6493-a8b2-7429-8dc8-d616a756c492" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromIssuanceOfSecuredDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6493-a8b2-7e59-af40-29f9f82e7ba4" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfSecuredDebt_019e6493-a8b2-7429-8dc8-d616a756c492" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019e6493-a8b2-7750-9e5b-3042c8581186" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6493-a8b2-7e59-af40-29f9f82e7ba4" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019e6493-a8b2-7750-9e5b-3042c8581186" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_DebtInstrumentFloorInterestRate_019e6493-a8b2-790f-9a81-6a429d603677" xlink:href="kop-20260630.xsd#kop_DebtInstrumentFloorInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6493-a8b2-7e59-af40-29f9f82e7ba4" xlink:to="loc_kop_DebtInstrumentFloorInterestRate_019e6493-a8b2-790f-9a81-6a429d603677" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_DebtInstrumentPercentageOfPrincipalPayment_019e6493-a8b2-75ec-a52d-09d8e753ade5" xlink:href="kop-20260630.xsd#kop_DebtInstrumentPercentageOfPrincipalPayment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e6493-a8b2-7e59-af40-29f9f82e7ba4" xlink:to="loc_kop_DebtInstrumentPercentageOfPrincipalPayment_019e6493-a8b2-75ec-a52d-09d8e753ade5" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.koppers.com/role/CommitmentsandContingentLiabilitiesAdditionalInformationDetails" xlink:type="simple" xlink:href="kop-20260630.xsd#CommitmentsandContingentLiabilitiesAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.koppers.com/role/CommitmentsandContingentLiabilitiesAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesLineItems_019e6493-a8b2-7fba-8c12-cdb9a3e0044b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingenciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesTable_019e6493-a8b2-717c-9b61-78454a43847e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingenciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e6493-a8b2-7fba-8c12-cdb9a3e0044b" xlink:to="loc_us-gaap_LossContingenciesTable_019e6493-a8b2-717c-9b61-78454a43847e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_019e6493-a8b2-7b81-9051-c6667a4a674e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019e6493-a8b2-717c-9b61-78454a43847e" xlink:to="loc_srt_CounterpartyNameAxis_019e6493-a8b2-7b81-9051-c6667a4a674e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019e6493-a8b2-7b81-9051-c6667a4a674e_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_019e6493-a8b2-7b81-9051-c6667a4a674e" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019e6493-a8b2-7b81-9051-c6667a4a674e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019e6493-a8b2-7ac4-a9d4-32107f66ef64" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_019e6493-a8b2-7b81-9051-c6667a4a674e" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019e6493-a8b2-7ac4-a9d4-32107f66ef64" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_BeazerEastMember_019e6493-a8b2-7f72-919b-2a491a6bfa04" xlink:href="kop-20260630.xsd#kop_BeazerEastMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019e6493-a8b2-7ac4-a9d4-32107f66ef64" xlink:to="loc_kop_BeazerEastMember_019e6493-a8b2-7f72-919b-2a491a6bfa04" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_019e6493-a8b2-7e01-a947-75d1822da391" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019e6493-a8b2-717c-9b61-78454a43847e" xlink:to="loc_srt_StatementGeographicalAxis_019e6493-a8b2-7e01-a947-75d1822da391" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019e6493-a8b2-7e01-a947-75d1822da391_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_019e6493-a8b2-7e01-a947-75d1822da391" xlink:to="loc_srt_SegmentGeographicalDomain_019e6493-a8b2-7e01-a947-75d1822da391_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019e6493-a8b2-795d-baf9-e913cd94151b" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_019e6493-a8b2-7e01-a947-75d1822da391" xlink:to="loc_srt_SegmentGeographicalDomain_019e6493-a8b2-795d-baf9-e913cd94151b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_stpr_OR_019e6493-a8b2-70cd-8705-490604dad566" xlink:href="https://xbrl.sec.gov/stpr/2026/stpr-2026.xsd#stpr_OR"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019e6493-a8b2-795d-baf9-e913cd94151b" xlink:to="loc_stpr_OR_019e6493-a8b2-70cd-8705-490604dad566" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_019e6493-a8b2-7b21-8302-12ed604733b8" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019e6493-a8b2-795d-baf9-e913cd94151b" xlink:to="loc_country_US_019e6493-a8b2-7b21-8302-12ed604733b8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_AU_019e6493-a8b2-7370-8eb6-e9f2e2ad0037" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_AU"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019e6493-a8b2-795d-baf9-e913cd94151b" xlink:to="loc_country_AU_019e6493-a8b2-7370-8eb6-e9f2e2ad0037" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseAxis_019e6493-a8b2-7e0c-b077-2098e96287b5" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_LitigationCaseAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019e6493-a8b2-717c-9b61-78454a43847e" xlink:to="loc_srt_LitigationCaseAxis_019e6493-a8b2-7e0c-b077-2098e96287b5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_019e6493-a8b2-7e0c-b077-2098e96287b5_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_LitigationCaseAxis_019e6493-a8b2-7e0c-b077-2098e96287b5" xlink:to="loc_srt_LitigationCaseTypeDomain_019e6493-a8b2-7e0c-b077-2098e96287b5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_019e6493-a8b2-7b09-bf76-f116405ab358" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_LitigationCaseAxis_019e6493-a8b2-7e0c-b077-2098e96287b5" xlink:to="loc_srt_LitigationCaseTypeDomain_019e6493-a8b2-7b09-bf76-f116405ab358" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_PortlandHarborMember_019e6493-a8b2-752f-b4fc-d2df59bd71ce" xlink:href="kop-20260630.xsd#kop_PortlandHarborMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_019e6493-a8b2-7b09-bf76-f116405ab358" xlink:to="loc_kop_PortlandHarborMember_019e6493-a8b2-752f-b4fc-d2df59bd71ce" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_PortlandHarborTrusteeClaimsMember_019e6493-a8b2-7122-a54e-dde687288ea6" xlink:href="kop-20260630.xsd#kop_PortlandHarborTrusteeClaimsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_019e6493-a8b2-7b09-bf76-f116405ab358" xlink:to="loc_kop_PortlandHarborTrusteeClaimsMember_019e6493-a8b2-7122-a54e-dde687288ea6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EnvironmentalRemediationSiteAxis_019e6493-a8b2-7e33-9ae6-3ffbbe1759a0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EnvironmentalRemediationSiteAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019e6493-a8b2-717c-9b61-78454a43847e" xlink:to="loc_us-gaap_EnvironmentalRemediationSiteAxis_019e6493-a8b2-7e33-9ae6-3ffbbe1759a0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EnvironmentalRemediationSiteDomain_019e6493-a8b2-7e33-9ae6-3ffbbe1759a0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EnvironmentalRemediationSiteDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EnvironmentalRemediationSiteAxis_019e6493-a8b2-7e33-9ae6-3ffbbe1759a0" xlink:to="loc_us-gaap_EnvironmentalRemediationSiteDomain_019e6493-a8b2-7e33-9ae6-3ffbbe1759a0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EnvironmentalRemediationSiteDomain_019e6493-a8b2-793e-b302-c8a6595e8cfe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EnvironmentalRemediationSiteDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EnvironmentalRemediationSiteAxis_019e6493-a8b2-7e33-9ae6-3ffbbe1759a0" xlink:to="loc_us-gaap_EnvironmentalRemediationSiteDomain_019e6493-a8b2-793e-b302-c8a6595e8cfe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_EnvironmentalRemedialSiteTwoMember_019e6493-a8b2-75b4-ac43-d26d6abb1cc5" xlink:href="kop-20260630.xsd#kop_EnvironmentalRemedialSiteTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EnvironmentalRemediationSiteDomain_019e6493-a8b2-793e-b302-c8a6595e8cfe" xlink:to="loc_kop_EnvironmentalRemedialSiteTwoMember_019e6493-a8b2-75b4-ac43-d26d6abb1cc5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e6493-a8b2-7ccb-8cdf-00e2c1d88453" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019e6493-a8b2-717c-9b61-78454a43847e" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e6493-a8b2-7ccb-8cdf-00e2c1d88453" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e6493-a8b2-7ccb-8cdf-00e2c1d88453_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e6493-a8b2-7ccb-8cdf-00e2c1d88453" xlink:to="loc_us-gaap_SegmentDomain_019e6493-a8b2-7ccb-8cdf-00e2c1d88453_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e6493-a8b2-79b3-a5b0-67132d2d8967" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e6493-a8b2-7ccb-8cdf-00e2c1d88453" xlink:to="loc_us-gaap_SegmentDomain_019e6493-a8b2-79b3-a5b0-67132d2d8967" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_PerformanceChemicalsMember_019e6493-a8b2-7e5e-ae77-5df593e8c937" xlink:href="kop-20260630.xsd#kop_PerformanceChemicalsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6493-a8b2-79b3-a5b0-67132d2d8967" xlink:to="loc_kop_PerformanceChemicalsMember_019e6493-a8b2-7e5e-ae77-5df593e8c937" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_UtilityAndIndustrialProductsMember_019e6493-a8b2-7440-9dcc-7e7d0b115ffa" xlink:href="kop-20260630.xsd#kop_UtilityAndIndustrialProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e6493-a8b2-79b3-a5b0-67132d2d8967" xlink:to="loc_kop_UtilityAndIndustrialProductsMember_019e6493-a8b2-7440-9dcc-7e7d0b115ffa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_NumberOfSitesListedOnNationalPrioritiesList_019e6493-a8b2-770d-b4bb-0ab868540a5c" xlink:href="kop-20260630.xsd#kop_NumberOfSitesListedOnNationalPrioritiesList"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e6493-a8b2-7fba-8c12-cdb9a3e0044b" xlink:to="loc_kop_NumberOfSitesListedOnNationalPrioritiesList_019e6493-a8b2-770d-b4bb-0ab868540a5c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_NumberOfPotentiallyResponsibleParties_019e6493-a8b2-717a-a84c-6dbfc0bcea92" xlink:href="kop-20260630.xsd#kop_NumberOfPotentiallyResponsibleParties"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e6493-a8b2-7fba-8c12-cdb9a3e0044b" xlink:to="loc_kop_NumberOfPotentiallyResponsibleParties_019e6493-a8b2-717a-a84c-6dbfc0bcea92" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_NetPresentValueOfSelectedRemedyEstimation_019e6493-a8b2-7517-a097-1a36a5a80872" xlink:href="kop-20260630.xsd#kop_NetPresentValueOfSelectedRemedyEstimation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e6493-a8b2-7fba-8c12-cdb9a3e0044b" xlink:to="loc_kop_NetPresentValueOfSelectedRemedyEstimation_019e6493-a8b2-7517-a097-1a36a5a80872" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_UndiscountedCostOfSelectedRemedyEstimation_019e6493-a8b2-7135-8c34-e3518e4c9a96" xlink:href="kop-20260630.xsd#kop_UndiscountedCostOfSelectedRemedyEstimation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e6493-a8b2-7fba-8c12-cdb9a3e0044b" xlink:to="loc_kop_UndiscountedCostOfSelectedRemedyEstimation_019e6493-a8b2-7135-8c34-e3518e4c9a96" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_LossContingencyClaimsNumber_019e6493-a8b2-7572-9588-74d6b10be869" xlink:href="kop-20260630.xsd#kop_LossContingencyClaimsNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e6493-a8b2-7fba-8c12-cdb9a3e0044b" xlink:to="loc_kop_LossContingencyClaimsNumber_019e6493-a8b2-7572-9588-74d6b10be869" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyClaimsSettledNumber_019e6493-a8b2-7eda-9673-6c039686ee8d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingencyClaimsSettledNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e6493-a8b2-7fba-8c12-cdb9a3e0044b" xlink:to="loc_us-gaap_LossContingencyClaimsSettledNumber_019e6493-a8b2-7eda-9673-6c039686ee8d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_LossContingencyNumberOfPlaintiffsFilingAnAppeal_019e6493-a8b2-7080-9a51-f875cb5957ef" xlink:href="kop-20260630.xsd#kop_LossContingencyNumberOfPlaintiffsFilingAnAppeal"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e6493-a8b2-7fba-8c12-cdb9a3e0044b" xlink:to="loc_kop_LossContingencyNumberOfPlaintiffsFilingAnAppeal_019e6493-a8b2-7080-9a51-f875cb5957ef" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_CostsOfParticipatingInPotentiallyResponsiblePartiesGroup_019e6493-a8b2-7b2f-acd5-42123d08c544" xlink:href="kop-20260630.xsd#kop_CostsOfParticipatingInPotentiallyResponsiblePartiesGroup"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e6493-a8b2-7fba-8c12-cdb9a3e0044b" xlink:to="loc_kop_CostsOfParticipatingInPotentiallyResponsiblePartiesGroup_019e6493-a8b2-7b2f-acd5-42123d08c544" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_kop_NumberOfAcquiredPlantSitesWithRecordedEnvironmentalRemediationLiability_019e6493-a8b2-7c22-b285-4fca0d7ad2ad" xlink:href="kop-20260630.xsd#kop_NumberOfAcquiredPlantSitesWithRecordedEnvironmentalRemediationLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e6493-a8b2-7fba-8c12-cdb9a3e0044b" xlink:to="loc_kop_NumberOfAcquiredPlantSitesWithRecordedEnvironmentalRemediationLiability_019e6493-a8b2-7c22-b285-4fca0d7ad2ad" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccrualForEnvironmentalLossContingencies_019e6493-a8b2-7944-af29-b28b18242588" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccrualForEnvironmentalLossContingencies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e6493-a8b2-7fba-8c12-cdb9a3e0044b" xlink:to="loc_us-gaap_AccrualForEnvironmentalLossContingencies_019e6493-a8b2-7944-af29-b28b18242588" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedEnvironmentalLossContingenciesCurrent_019e6493-a8b2-7a2d-ac68-6735ee6c8c05" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccruedEnvironmentalLossContingenciesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e6493-a8b2-7fba-8c12-cdb9a3e0044b" xlink:to="loc_us-gaap_AccruedEnvironmentalLossContingenciesCurrent_019e6493-a8b2-7a2d-ac68-6735ee6c8c05" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EnvironmentalLossContingencyCurrentStatementOfFinancialPositionExtensibleEnumeration_019e6493-a8b2-7e0d-a3ca-6655415ab333" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EnvironmentalLossContingencyCurrentStatementOfFinancialPositionExtensibleEnumeration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccruedEnvironmentalLossContingenciesCurrent_019e6493-a8b2-7a2d-ac68-6735ee6c8c05" xlink:to="loc_us-gaap_EnvironmentalLossContingencyCurrentStatementOfFinancialPositionExtensibleEnumeration_019e6493-a8b2-7e0d-a3ca-6655415ab333" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fasb.org/srt/role/srt-eedm/ExtensibleEnumerationLists" xlink:type="simple" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-roles-2026.xsd#eedm1"/>
  <link:definitionLink xlink:role="http://fasb.org/srt/role/srt-eedm/ExtensibleEnumerationLists" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesCurrentAbstract"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrent" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract" xlink:to="loc_us-gaap_LiabilitiesCurrent" xlink:type="arc" order="41"/>
  </link:definitionLink>
</link:linkbase>
