The Collaborative International Dictionary of English v.0.48:

Legitimatize \Le*git"i*ma*tize\ (-t[imac]z), v. t.
   To legitimate.
   [1913 Webster]

WordNet (r) 3.0 (2006):

legitimatize
    v 1: make legal; "Marijuana should be legalized" [syn:
         legalize, legalise, decriminalize, decriminalise,
         legitimize, legitimise, legitimate, legitimatize,
         legitimatise] [ant: criminalise, criminalize,
         illegalise, illegalize, outlaw]