
Grispi (14-1) had been scheduled to face Erik Koch at WEC 52 on Nov. 11 until being redirected into the title opportunity. The 22-year-old Grispi is 14-1 overall and is unbeaten in four WEC fights, winning all four in the first round.
In his most recent bout, he defeated LC Davis by technical submission via guillotine choke.
The dynamic Aldo is 18-1 overall and 8-0 during his days in the WEC.
UFC 125 takes place on New Year's Day in Las Vegas. The main event of the show is a UFC lightweight championship fight between Frankie Edgar and Gray Maynard.
Aldo-Grispi will serve as the night's co-main event. Other bouts made official by the UFC include Clay Guida vs. Takanori Gomi, Chris Leben vs. Brian Stann, Nate Diaz vs. Dong Hyun Kim and Brandon Vera vs. Thiago Silva.