Two ex-UFC champions have signed on to fight each other in a jiu jitsu event in Vegas ... TMZ Sports has learned.
Frank Mir (former UFC heavyweight champ) will take on Forrest Griffin (former UFC light heavyweight champ) at the Black Belt CBD Invitational on October 20th.
Griffin hasn't taken a pro fight since July 2012 -- when he beat up Tito Ortiz at UFC 148.
As for Mir, he fought last April at Bellator 198 -- when he was defeated by Fedor Emelianenko in the Bellator Heavyweight Grand Prix tournament.