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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2018-10-01</periodOfReport>

    <notSubjectToSection16>1</notSubjectToSection16>

    <issuer>
        <issuerCik>0001028734</issuerCik>
        <issuerName>COBIZ FINANCIAL INC</issuerName>
        <issuerTradingSymbol>COBZ</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001555428</rptOwnerCik>
            <rptOwnerName>Hutchinson Michael G</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1401 LAWRENCE ST.</rptOwnerStreet1>
            <rptOwnerStreet2>STE. 1200</rptOwnerStreet2>
            <rptOwnerCity>DENVER</rptOwnerCity>
            <rptOwnerState>CO</rptOwnerState>
            <rptOwnerZipCode>80202</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>2018-10-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>299</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Shares disposed of pursuant to the Agreement and Plan of Merger, dated as of June 17, 2018 (the &quot;Merger Agreement&quot;), by and among CoBiz Financial Inc. (&quot;Issuer&quot;), and BOK Financial Corporation (&quot;BOKF Parent&quot;), and BOKF Merger Corporation Number Sixteen (&quot;Merger Sub&quot;), a wholly owned subsidiary of BOKF Parent, on October 1, 2018, Issuer was merged with and into Merger Sub, with Merger Sub continuing as the surviving corporation (the &quot;Merger&quot;).  At the time of the merger, each share of Issuer common stock was converted into the right to receive merger consideration consisting of (i) 0.17 shares of BOKF Parent common stock and (ii) $5.70 cash (&quot;Merger Consideration&quot;).</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Michael G Hutchinson</signatureName>
        <signatureDate>2018-10-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
