The Collaborative International Dictionary of English v.0.48:

Monopolist \Mo*nop"o*list\ (m[-o]*n[o^]p"[-o]*l[i^]st), n.
   One who monopolizes; one who has a monopoly; one who favors
   monopoly.
   [1913 Webster]

WordNet (r) 3.0 (2006):

monopolist
    n 1: someone who monopolizes the means of producing or selling
         something [syn: monopolist, monopolizer, monopoliser]