The Collaborative International Dictionary of English v.0.48:

Justificative \Jus*tif"i*ca*tive\ (?; 277), a. [Cf. F.
   justificatif.]
   Having power to justify; justificatory.
   [1913 Webster]

WordNet (r) 3.0 (2006):

justificative
    adj 1: attempting to justify or defend in speech or writing
           [syn: defensive, justificative, justificatory]
    2: providing justification [syn: justificative,
       justificatory, vindicatory]