Source : Webster's Revised Unabridged Dictionary (1913)

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

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

Source : WordNet®

secessionist
     n : an advocate of secessionism