The Collaborative International Dictionary of English v.0.48:

Bigamist \Big"a*mist\, n. [Cf. Digamist.]
   One who is guilty of bigamy. --Ayliffe.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bigamist
    n 1: someone who marries one person while already legally
         married to another