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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-03-12</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001166126</issuerCik>
        <issuerName>J C PENNEY CO INC</issuerName>
        <issuerTradingSymbol>JCP</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001246085</rptOwnerCik>
            <rptOwnerName>CAVANAUGH ROBERT B</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O J. C. PENNEY COMPANY, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>6501 LEGACY DRIVE</rptOwnerStreet2>
            <rptOwnerCity>PLANO</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>75024</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>EVP, Chief Financial Officer</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock of 50 cents Par Value</value>
            </securityTitle>
            <transactionDate>
                <value>2010-03-12</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>806</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>30.56</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>21908.915</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock of 50 cents Par Value</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5877</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Trustee of 401(k)</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Represents shares used to satisfy tax withholding obligation upon vesting of restricted stock units granted under the Company's 2005 Equity Compensation Plan.</footnote>
        <footnote id="F2">Includes 169.697 restricted stock units acquired since March 22, 2009 pursuant to dividend equivalents on restricted stock unit awards. Dividend equivalents on restricted stock unit awards accrue on a quarterly basis, exempt under Rule 16a-11.</footnote>
        <footnote id="F3">Represents equivalent shares based on units of participation in the JCPenney stock fund allocated to Mr. Cavanaugh's account in the Company's 401(k) plan as of March 1, 2010. The shares of JCPenney common stock in the fund are held by the trustee of the 401(k) plan. Changes in the amount of securities beneficially owned reflect changes in the value of the fund, the number of units of participation in the fund held by all participants, and the number of units of participation held by Mr. Cavanaugh.</footnote>
    </footnotes>

    <remarks>***Under Power of attorney as filed herewith.

Exhibit List:
Exhibit No. 24 Power of Attorney (POA)</remarks>

    <ownerSignature>
        <signatureName>*** /s/ Salil R. Virkar, attorney in fact</signatureName>
        <signatureDate>2010-03-16</signatureDate>
    </ownerSignature>
</ownershipDocument>
