The Collaborative International Dictionary of English v.0.48:

Polygamist \Po*lyg"a*mist\, n. [Cf. F. polygamiste, polygame,
   Gr. ?, a.]
   One who practices polygamy, or maintains its lawfulness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

polygamist
    n 1: someone who is married to two or more people at the same
         time