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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2016-03-08</periodOfReport>

    <issuer>
        <issuerCik>0001011006</issuerCik>
        <issuerName>YAHOO INC</issuerName>
        <issuerTradingSymbol>YHOO</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001190639</rptOwnerCik>
            <rptOwnerName>BRANDT ERIC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O YAHOO! INC.</rptOwnerStreet1>
            <rptOwnerStreet2>701 FIRST AVENUE</rptOwnerStreet2>
            <rptOwnerCity>SUNNYVALE</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94089</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2016-03-08</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>2156</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2156</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">These shares represent restricted stock units granted automatically under the Yahoo! Inc. (&quot;Yahoo&quot;) Directors' Stock Plan on the date the reporting person first became a director.  Each restricted stock unit represents the contingent right to receive one share of Yahoo common stock. These units are scheduled to vest in two equal installments on March 24, 2016 and June 24, 2016 (or, if earlier, the day before Yahoo's next annual meeting of shareholders), subject to the reporting person's continuous service as a director. Vested shares will generally be delivered on the earlier of the last vesting date and the date the director's service terminates, subject to any election by the reporting person to defer payment.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Eric K. Brandt</signatureName>
        <signatureDate>2016-03-10</signatureDate>
    </ownerSignature>
</ownershipDocument>
