Ireland's Ian Machado Garry will face fellow undefeated welterweight Shavkat Rakhmonov at UFC 310 on 7 December in Las Vegas.
Machado Garry, 26, steps in to face Rakhmonov after welterweight champion Belal Muhammad withdrew from the bout because of an injury.
The contest will be the co-main event, meaning it will be five rounds.
It is the first time in Machado Garry's career he will fight for five rounds in the UFC and gives the Dubliner the chance to strip Rakhmonov of his position as number one contender to the title.
"I'm very disappointed in our division," Rakhmonov posted on X, claiming Leon Edwards and Colby Covington turned down the opportunity to fight him.
"The only one who backed up his words is Ian, and I respect that. But make no mistake, I'm coming for your 0 and will fight for the belt after."
Machado Garry is undefeated in 15 fights, with his most recent win coming on points over Briton Michael 'Venom' Page in June.
That victory was by far Machado Garry's biggest to date, but in Rakhmonov he faces another huge step up in quality.
Kazakhstan's Rakhmonov, 30, has 18 wins on his MMA record and has never been taken the distance. He has 10 submission victories and eight knockouts.
No Irish fighter has fought for a UFC title since Conor McGregor lost to Khabib Nurmagomedov in 2018.