Rousey, a former Olympic judo medalist, was not competitive in the fight at T-Mobile Arena, her first since losing her belt to Holly Holm in November 2015, her first career defeat. Rousey became arguably the world’s most famous female athlete and a combat sports trailblazer as she rocketed to the top of the U.F.C., but a year away from the sport apparently did little to heal the holes in her game.

Nunes, a veteran from Brazil, claimed her belt with a violent stoppage of Miesha Tate at U.F.C. 200 in July, completing her unlikely ascent. Nunes lost three fights earlier in her career to opponents beaten easily by Rousey, but Nunes’s work ethic and toughness propelled her into the fight against Rousey.

Rousey had not fought since the November 2015 fight, when Holm stopped her with a head kick in one of the biggest upsets in mixed martial arts history. Rousey had risen to prominence with a series of one-sided victories, but Holm’s veteran striking made Rousey look amateurish.

Rousey nearly vanished from public view after her loss, taking time away from the gym and developing her acting career. After the bantamweight belt changed hands two times in her absence, Rousey agreed to return for a shot at reclaiming her title.