<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2007-07-13</periodOfReport>

    <notSubjectToSection16>1</notSubjectToSection16>

    <issuer>
        <issuerCik>0001236736</issuerCik>
        <issuerName>GENESIS HEALTHCARE CORP</issuerName>
        <issuerTradingSymbol>GHCI</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001235869</rptOwnerCik>
            <rptOwnerName>DEPODESTA JOHN</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1700 OLD MEADOW ROAD</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>MCLEAN</rptOwnerCity>
            <rptOwnerState>VA</rptOwnerState>
            <rptOwnerZipCode>22102</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2007-07-13</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>12245</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>69.35</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                    <footnoteId id="F1"/>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Pursuant to the Merger Agreement, dated as of January 15, 2007, as amended, by and among Genesis HealthCare Corporation, FC-GEN Acquisition, Inc. and GEN Acquisition Corp., a wholly owned subsidiary of FC-GEN Acquisition, Inc., 12,245 shares of restricted common stock, which became fully vested upon consummation of the merger, were disposed of in exchange for a cash payment of $69.35 per share.</footnote>
    </footnotes>

    <remarks>The filing of this Statement shall not be construed as an admission (a) that the person filing this Statement is, for the purposes of Section 16 of the Securities Exchange Act of 1934 (as amended), the beneficial owner of any equity securities covered by this Statement, or (b) that this Statement is legally required to be filed by such person.</remarks>

    <ownerSignature>
        <signatureName>/s/ Richard E. Reilly, Attorney-in-fact</signatureName>
        <signatureDate>2007-07-13</signatureDate>
    </ownerSignature>
</ownershipDocument>
