<?xml version="1.0"?>

<ownershipDocument>

    <schemaVersion>X0201</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2004-01-02</periodOfReport>


    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001135971</issuerCik>
        <issuerName>PEPCO HOLDINGS INC</issuerName>
        <issuerTradingSymbol>POM</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001032012</rptOwnerCik>
            <rptOwnerName>MCKENZIE FLORETTA D</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>THE MCKENZIE GROUP, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>1101 - 17TH STREET, NW, SUITE 1200</rptOwnerStreet2>
            <rptOwnerCity>WASHINGTON</rptOwnerCity>
            <rptOwnerState>DC</rptOwnerState>
            <rptOwnerZipCode>20036</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>

        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2004-01-02</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>194</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4962</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>

    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Acquired in consideration for services, with the number of shares of phantom stock calculated by dividing the amount of meeting fees and/or retainer payable ($1,250 per meeting or that portion of the annual retainer of $30,000 which the director elected to receive in phantom stock) by the market price of the Pepco Holdings, Inc. (PHI) shares at the close of business two business days before the meeting or retainer payment date, respectively.  At least one-half of the retainer is required to be paid in common stock or phantom stock.</footnote>
    </footnotes>


    <ownerSignature>
        <signatureName>Floretta D. McKenzie by Ellen Sheriff Rogers, Attorney-in-Fact</signatureName>
        <signatureDate>2004-01-05</signatureDate>
    </ownerSignature>

</ownershipDocument>

