Frank Mir Fighting Forrest Griffin In Jiu-Jitsu Tourney

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 20.

Griffin hasn't taken a pro fight since July 2012 -- when he beat up Tito Ortiz at UFC 148.

As for Mir, he fought in April at Bellator 198 -- when he was defeated by Fedor Emelianenko in the Bellator Heavyweight Grand Prix tournament.

As for the CBD Invitational, we're told Ana Vieira, Catherine Perret, Bia Masquita and Luiza Moteiro are also fighting on the card.

Event organizers are touting the fight card as the first of its kind to offer the same purse for male and female fighters.

"We’re a woman-founded and woman-led company, born and bred in Las Vegas, and Jiu Jitsu is a huge part of our culture and even my family – my daughter loves to train Jiu Jitsu,” Black Belt CBD CEO and founder Krista Whitley said.

"We specialize in CBD-based performance-training products made for serious athletes of all genders, and when we launched the Black Belt CBD Invitational, it was only natural that we bring this philosophy into the Jiu Jitsu community."