The Collaborative International Dictionary of English v.0.48:

Internationalist \In`ter*na"tion*al*ist\, n.
   1. One who is versed in the principles of international law.
      [1913 Webster]

   2. A member of the International; one who believes in, or
      advocates the doctrines of, the International.
      [1913 Webster]

WordNet (r) 3.0 (2006):

internationalist
    adj 1: influenced by or advocating internationalism [syn:
           internationalist, internationalistic]
    n 1: an advocate of internationalism
    2: a member of a socialist or communist international