The Collaborative International Dictionary of English v.0.48:

Secessionist \Se*ces"sion*ist\, n.
   1. One who upholds secession.
      [1913 Webster]

   2. (U.S. Hist.) One who holds to the belief that a State has
      the right to separate from the Union at its will.
      [1913 Webster]

WordNet (r) 3.0 (2006):

secessionist
    n 1: an advocate of secessionism