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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2009-08-11</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001324410</issuerCik>
        <issuerName>Guaranty Bancorp</issuerName>
        <issuerTradingSymbol>GBNK</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0000949197</rptOwnerCik>
            <rptOwnerName>WYCOFF W KIRK</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1331 17TH STREET, STE. 300</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>DENVER</rptOwnerCity>
            <rptOwnerState>CO</rptOwnerState>
            <rptOwnerZipCode>80202</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Series A Convertible Preferred Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>1.80</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2009-08-11</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>20000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>1000.00</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>11111111</value>
                    <footnoteId id="F3"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>20000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Patriot Financial Partners, L.P. and Patriot Financial Partners Parallel, L.P.</value>
                    <footnoteId id="F4"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Each share of Series A Convertible Preferred Stock will automatically convert into shares of common stock on August 11, 2014 and may be converted into shares of common stock following the earlier of August 11, 2011 or the occurrence of certain events resulting in the conversion, exchange or reclassification of the common stock.  Each share of Series A Convertible Preferred Stock is convertible into shares of common stock at a conversion price of $1.80 per share, adjustable to as low as $1.50 per share in the event of certain nonpayments of dividends (whether paid in cash or in kind) on the Series A Convertible Preferred Stock, and is also subject to customary anti-dilution adjustments.  The holders of the Series A Convertible Preferred Stock will vote together with the holders of common stock as a single class on all matters upon which the holders of common stock are entitled to vote.</footnote>
        <footnote id="F2">Each share of Series A Convertible Preferred Stock will be entitled to such number of votes as the number of shares of common stock into which such share of Series A Convertible Preferred Stock is convertible, assuming for voting purposes only, a conversion price of $2.00 per share.</footnote>
        <footnote id="F3">Assumes the conversion of the Series A Convertible Preferred Stock into common stock based upon a conversion price of $1.80 per share.</footnote>
        <footnote id="F4">Patriot Financial Partners, GP, L.P. (&quot;Patriot GP&quot;) is the sole general partner of each of Patriot Financial Partners, L.P. and Patriot Financial Partners Parallel, L.P. (together, the &quot;Funds&quot;). Accordingly, securities owned by the Funds may be regarded as being beneficially owned by Patriot GP. Patriot Financial Partners GP, LLC (&quot;Patriot LLC&quot;) is the general partner of Patriot GP. Accordingly, securities owned or deemed to be owned by Patriot GP may be regarded as being beneficially owned by Patriot LLC. W. Kirk Wycoff is a general partner of the Funds and Patriot GP and a member of Patriot LLC. Mr. Wycoff disclaims beneficial ownership of the reported securities except to the extent of his pecuniary interest therein, and this report shall not be deemed an admission that he is a beneficial owner of such securities for the purposes of Section 16.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>By: /s/ Zsolt K. Bessko for: W. Kirk Wycoff</signatureName>
        <signatureDate>2009-08-12</signatureDate>
    </ownerSignature>
</ownershipDocument>
