The Collaborative International Dictionary of English v.0.48:

Monotheist \Mon"o*the*ist\, n. [Cf. F. monoth['e]iste.]
   One who believes that there is but one God.
   [1913 Webster]

WordNet (r) 3.0 (2006):

monotheist
    n 1: a believer in one god

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "monotheist":
   allotheist, anthropolater, anthropotheist, autotheist, cosmotheist,
   deist, ditheist, dualist, henotheist, hylotheist, multitheist,
   myriotheist, pantheist, physitheist, polytheist, psychotheist,
   tetratheist, theist, theriotheist, tritheist, zootheist