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

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "secessionist":
   abjuratory, apostate, backslider, bolter, breakaway,
   collaborationist, collaborator, convert, defector, deserter,
   fifth columnist, mugwump, mugwumpian, mugwumpish, proselyte,
   quisling, recidivist, recreant, renegade, renegado, renegate,
   renunciative, renunciatory, repudiative, repudiatory, reversionist,
   revocatory, runagate, schismatic, seceder, separatist,
   strikebreaker, tergiversant, tergiversator, traitor, turnabout,
   turncoat, turntail