<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0101</schemaVersion>

    <documentType>4</documentType>
<periodOfReport>2003-07-17</periodOfReport>
<notSubjectToSection16>0</notSubjectToSection16>
    <issuer>
        <issuerCik>0000703970</issuerCik>
        <issuerName>ALLEGIANT BANCORP INC/MO/</issuerName>
        <issuerTradingSymbol>ALLE</issuerTradingSymbol>
</issuer>
<reportingOwner>
  <reportingOwnerId>
            <rptOwnerCik>0001055099</rptOwnerCik>
            <rptOwnerName>FELMAN LEON A</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>  </rptOwnerStreet1>
            <rptOwnerStreet2>  </rptOwnerStreet2>
            <rptOwnerCity>  </rptOwnerCity>
            <rptOwnerState>  </rptOwnerState>
            <rptOwnerZipCode>  </rptOwnerZipCode>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
    <isDirector>1</isDirector>
    <isOfficer>0</isOfficer>
    <isTenPercentOwner>0</isTenPercentOwner>
    <isOther>0</isOther>
    <officerTitle></officerTitle>
    <otherText></otherText>
  </reportingOwnerRelationship>
</reportingOwner>
<nonDerivativeSecurity>
  <securityTitle>
    <value>Common Stock</value>
  </securityTitle>
  <transactionDate>
    <value>2003-07-17</value>
  </transactionDate>
  <transactionCoding>
    <transactionFormType>4</transactionFormType>
    <transactionCode>M</transactionCode>
    <equitySwapInvolved>0</equitySwapInvolved>
  </transactionCoding>
  <transactionTimeliness>
    <value></value>
  </transactionTimeliness>
  <transactionAmounts>
    <transactionShares>
      <value>5000</value>
    </transactionShares>
    <transactionValue>
      <value>101650.00</value>
    </transactionValue>
    <transactionAcquiredDisposedCode>
      <value>A</value>
    </transactionAcquiredDisposedCode>
  </transactionAmounts>
  <postTransactionAmounts>
    <sharesOwnedFollowingTransaction>
      <value>1349074.5477</value>
    </sharesOwnedFollowingTransaction>
  </postTransactionAmounts>
  <ownershipNature>
    <directOrIndirectOwnership>
      <value>D</value>
    </directOrIndirectOwnership>
    <natureOfOwnership>
      <value></value>
    </natureOfOwnership>
  </ownershipNature>
</nonDerivativeSecurity>
<derivativeSecurity>
  <securityTitle>
    <value>Common Stock</value>
  </securityTitle>
  <conversionOrExercisePrice>
    <value>20.33</value>
  </conversionOrExercisePrice>
  <transactionDate>
    <value>2003-07-17</value>
  </transactionDate>
  <transactionCoding>
    <transactionFormType>4</transactionFormType>
    <transactionCode>M</transactionCode>
    <equitySwapInvolved>0</equitySwapInvolved>
  </transactionCoding>
  <transactionTimeliness>
    <value></value>
  </transactionTimeliness>
  <transactionAmounts>
    <transactionShares>
      <value>5000</value>
    </transactionShares>
    <transactionValue>
      <value>101650.00</value>
    </transactionValue>
    <transactionAcquiredDisposedCode>
      <value>D</value>
    </transactionAcquiredDisposedCode>
  </transactionAmounts>
  <exerciseDate>
    <value>2003-05-09</value>
  </exerciseDate>
  <expirationDate>
    <value>2008-04-30</value>
  </expirationDate>
  <underlyingSecurity>
    <underlyingSecurityTitle>
      <value>Common Stock</value>
    </underlyingSecurityTitle>
    <underlyingSecurityShares>
      <value>5000</value>
    </underlyingSecurityShares>
  </underlyingSecurity>
  <postTransactionAmounts>
    <sharesOwnedFollowingTransaction>
      <value>0</value>
    </sharesOwnedFollowingTransaction>
  </postTransactionAmounts>
  <ownershipNature>
    <directOrIndirectOwnership>
      <value>D</value>
    </directOrIndirectOwnership>
    <natureOfOwnership>
      <value></value>
    </natureOfOwnership>
  </ownershipNature>
</derivativeSecurity>
<ownerSignature>
  <signatureName>Leon A. Felman</signatureName>
  <signatureDate>2003-07-18</signatureDate>
</ownerSignature>
</ownershipDocument>

