Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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