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

    <schemaVersion>X0304</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2012-02-29</periodOfReport>

    <issuer>
        <issuerCik>0001142701</issuerCik>
        <issuerName>UNITED ONLINE INC</issuerName>
        <issuerTradingSymbol>UNTD</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001364748</rptOwnerCik>
            <rptOwnerName>ZEITZ HAROLD A.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O UNITED ONLINE</rptOwnerStreet1>
            <rptOwnerStreet2>21301 BURBANK BOULVARD</rptOwnerStreet2>
            <rptOwnerCity>WOODLAND HILLS</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>91367</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isOfficer>1</isOfficer>
            <officerTitle>President, Memory Lane, Inc.</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2012-02-29</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>60000</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>172500</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Represents an award of restricted stock units. Each unit will entitle the Reporting Person to one share of the Issuer's common stock upon vesting. The units will vest and become issuable in three successive equal annual installments on February 15, 2013, February 15, 2014 and February 15, 2015, provided the Reporting Person continues in the Issuer's service through each applicable vesting date.</footnote>
        <footnote id="F2">Includes 112,500 shares subject to a restricted stock unit award granted on February 15, 2011 that will be issued as those units vest.</footnote>
        <footnote id="F3">Includes 60,000 shares subject to the reported restricted stock unit award.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Harold A. Zeitz</signatureName>
        <signatureDate>2012-03-02</signatureDate>
    </ownerSignature>
</ownershipDocument>
