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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-07-30</periodOfReport>

    <issuer>
        <issuerCik>0001227930</issuerCik>
        <issuerName>ENTROPIC COMMUNICATIONS INC</issuerName>
        <issuerTradingSymbol>ENTR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001005563</rptOwnerCik>
            <rptOwnerName>BARUCH THOMAS R</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O CMEA CAPITAL</rptOwnerStreet1>
            <rptOwnerStreet2>ONE EMBARCADERO CENTER, SUITE 3250</rptOwnerStreet2>
            <rptOwnerCity>SAN FRANCISCO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94111</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>2010-07-30</value>
            </transactionDate>
            <deemedExecutionDate>
                <value>2010-07-30</value>
            </deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1497261</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>7.83</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4512673</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By CMEA Ventures Information Technology II, L.P. and CMEA Ventures Information Technology II, CLP</value>
                    <footnoteId id="F3"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2010-07-30</value>
            </transactionDate>
            <deemedExecutionDate>
                <value>2010-07-30</value>
            </deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F4"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>6963</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>7.83</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>83959</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Pro rata distribution of shares.</footnote>
        <footnote id="F2">Consists of 1,336,485 shares being disposed of by CMEA Ventures Information Technology II, L.P. and 167,739 shares being disposed of by CMEA Ventures Information Technology II, Civil Law Partnership, less the 6,963 shares being distributed to Thomas R. Baruch.</footnote>
        <footnote id="F3">Consists of securities held directly by CMEA Ventures Information Technology II, L.P. and CMEA Ventures Information Techonology II, Civil Law Partnership (collectively &quot;CMEA 4&quot;).  CMEA Ventures IT Management II, L.P. (&quot;CMEA 4 GP&quot;) is the sole general partner of CMEA 4. Thomas R. Baruch (&quot;Baruch&quot;) is a general partner of CMEA 4 GP.  CMEA 4 GP and Baruch may be deemed to beneficially own the shares held by CMEA 4, but each of CMEA 4 and Baruch disclaims beneficial ownership of such shares, except to the extent of its pecuniary interest therein.</footnote>
        <footnote id="F4">Acquisition of a pro rata share of distribution.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Thomas R. Baruch</signatureName>
        <signatureDate>2010-08-24</signatureDate>
    </ownerSignature>
</ownershipDocument>
