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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2018-03-20</periodOfReport>

    <notSubjectToSection16>1</notSubjectToSection16>

    <issuer>
        <issuerCik>0000906193</issuerCik>
        <issuerName>KEY TECHNOLOGY INC</issuerName>
        <issuerTradingSymbol>KTEC</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001343599</rptOwnerCik>
            <rptOwnerName>Averick Robert M</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O KOKINO LLC</rptOwnerStreet1>
            <rptOwnerStreet2>201 TRESSER BOULEVARD, 3RD FLOOR</rptOwnerStreet2>
            <rptOwnerCity>STAMFORD</rptOwnerCity>
            <rptOwnerState>CT</rptOwnerState>
            <rptOwnerZipCode>06901</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2018-03-20</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>U</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>25000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>26.75</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Robert Averick also reports beneficial ownership of the Issuer's common stock pursuant to Rule 13d-1 of the Securities Exchange Act of 1934, as amended, pursuant to a Schedule 13D filed jointly by Leslie J. Schreyer, as Trustee under Trust Agreement dated December 23, 1989 FBO the issue of Jonathan D. Sackler (the &quot;Trust&quot;), M3C Holdings LLC (&quot;M3C&quot;), Mr. Averick and Piton Capital Partners LLC (&quot;Piton&quot;), with the SEC on June 30, 2016, as amended by a Schedule 13D Amendment No. 1 filed on January 31, 2018. The Trust, M3C and Mr. Averick are members of Piton. Piton's managing member is Piton Capital Management LLC (&quot;PCM&quot;). PCM's managing member is Kokino LLC (&quot;Kokino&quot;), and Kokino is Piton's trading manager. Mr. Averick manages the Trust's, M3C's and Piton's investments in the Issuer as Kokino's Portfolio Manager. Mr. Averick holds an economic interest in Piton (which interest may be held directly and, from time to time, indirectly through PCM).</footnote>
        <footnote id="F2">Mr. Averick's incentive compensation as an employee of Kokino, which is calculated in Kokino's discretion, may be based, among other things, on the performance of Shares held by the Trust, M3C and Piton. Such compensation may be paid in cash and/or by way of increasing Mr. Averick's minority interest in Piton (either directly or indirectly through PCM).</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Robert M. Averick</signatureName>
        <signatureDate>2018-03-22</signatureDate>
    </ownerSignature>
</ownershipDocument>
