The Collaborative International Dictionary of English v.0.48:

Polytheistic \Pol`y*the*is"tic\, Polytheistical
\Pol`y*the*is"tic*al\, a.
   Of or pertaining to polytheism; characterized by polytheism;
   professing or advocating polytheism; as, polytheistic
   worship; a polytheistic author, or nation. --
   Pol`y*the*is"tic*al*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

polytheistic
    adj 1: worshipping or believing in more than one god [ant:
           monotheistic]