The Collaborative International Dictionary of English v.0.48:

Imperialist \Im*pe"ri*al*ist\, n. [Cf. F. imp['e]rialiste.]
   One who serves an emperor; one who favors imperialism.
   [1913 Webster]

WordNet (r) 3.0 (2006):

imperialist
    adj 1: of or relating to imperialism; "imperialistic wars" [syn:
           imperialistic, imperialist]
    n 1: a believer in imperialism

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "imperialist":
   Bircher, Bourbon, Tory, conservatist, conservative, diehard,
   extreme right-winger, hard hat, monarchist, radical right,
   reactionarist, reactionary, reactionist, right, right wing,
   right-winger, rightist, royalist, social Darwinist, standpat,
   standpatter, ultraconservative