<SEC-DOCUMENT>0000045876-21-000070.txt : 20210421
<SEC-HEADER>0000045876-21-000070.hdr.sgml : 20210421
<ACCEPTANCE-DATETIME>20210421133806
ACCESSION NUMBER:		0000045876-21-000070
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20210420
FILED AS OF DATE:		20210421
DATE AS OF CHANGE:		20210421

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			HAZNEDAR CAROLANN I
		CENTRAL INDEX KEY:			0001756619

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-03970
		FILM NUMBER:		21840789

	MAIL ADDRESS:	
		STREET 1:		350 POPLAR CHURCH ROAD
		CITY:			CAMP HILL
		STATE:			PA
		ZIP:			17011

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			HARSCO CORP
		CENTRAL INDEX KEY:			0000045876
		STANDARD INDUSTRIAL CLASSIFICATION:	SERVICES-SERVICES, NEC [8900]
		IRS NUMBER:				231483991
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		350 POPLAR CHURCH ROAD
		CITY:			CAMP HILL
		STATE:			PA
		ZIP:			17011
		BUSINESS PHONE:		7177637064

	MAIL ADDRESS:	
		STREET 1:		350 POPLAR CHURCH ROAD
		CITY:			CAMP HILL
		STATE:			PA
		ZIP:			17011
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>wf-form4_161902666982561.xml
<DESCRIPTION>FORM 4
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2021-04-20</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000045876</issuerCik>
        <issuerName>HARSCO CORP</issuerName>
        <issuerTradingSymbol>HSC</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001756619</rptOwnerCik>
            <rptOwnerName>HAZNEDAR CAROLANN I</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>350 POPLAR CHURCH ROAD</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>CAMP HILL</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>17011</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Restricted Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2021-04-20</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>11662</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F2"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>11662.0</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Phantom Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F3"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2021-04-20</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>11662</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F3"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>11662.0</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>16399</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Restricted Stock Units convert into common stock on a one-for-one basis.</footnote>
        <footnote id="F2">On May 15, 2020, the reporting person was granted 11,662 restricted stock units that vested on the earlier of (i) the first anniversary of the grant date or (ii) the issuer's annual meeting of stockholders in the year immediately following the year of the grant date.</footnote>
        <footnote id="F3">Represents deferred compensation under the 2016 Non-Employee Directors' Long-Term Equity Compensation Plan. Each phantom stock unit represents the right to be paid in cash an amount equal to the fair market value of one share of Harsco Corporation Common Stock at the date of settlement. Represents a scheduled settlement date for the phantom stock units to be paid in cash in a lump sum commencing within thirty days at the begin of the calendar year 2031.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Carolann I. Haznedar</signatureName>
        <signatureDate>2021-04-21</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
