Rousey staggered and stumbled backward while Nunes relentlessly pursued her and landed multiple shots. Referee Herb Dean stopped the bout with Rousey still on her feet, and Rousey briefly protested the stoppage before leaving the cage in her mother's arms.
Cody Garbrandt also won the men's bantamweight title in the penultimate bout, battering champion Dominick Cruz to earn a surprising decision victory at T-Mobile Arena in the UFC's traditional end-of-the-year show in its hometown.
Rousey couldn't compete in her first fight since losing her belt to Holly Holm 13 months ago in her first career defeat. Rousey became arguably the world's most famous female athlete and a combat sports trailblazer while she rocketed atop the UFC, but a year away from the sport apparently did little to heal the holes in her game.
Nunes claimed her belt with a violent stoppage of Miesha Tate at UFC 200 in July, completing an unlikely ascent for a Brazilian veteran whose skills have improved in recent years. Nunes lost three fights earlier in her career to opponents beaten easily by Rousey, but Nunes' work ethic and toughness propelled her into the biggest fight in the women's game against Rousey.
Rousey hadn't fought since November 2015, when Holm stopped her with a head kick in one of the biggest upsets in MMA history. Rousey had never lost or been in much trouble while the former Olympic judo medalist rocketed to the top of her sport with a series of one-sided victories, but Holm's veteran striking made Rousey look amateurish before the dramatic finish.
Rousey nearly vanished from public view after her loss, taking time away from the gym and attending to her acting career. After the bantamweight belt changed hands two more times in her absence, Rousey agreed to return for a shot at reclaiming her title. The UFC also was sold during her absence to WME-IMG, the entertainment conglomerate that also represents Rousey's career.
But Rousey refused to promote this pay-per-view show, leaving Cruz and Nunes to do most of the work. While Rousey's media blackout will hurt her cut of the pay-per-view revenue, it didn't affect her guaranteed payday of $3 million, matching Conor McGregor for the biggest disclosed check in UFC history.
Earlier, Garbrandt (11-0) remained unbeaten with a virtuoso performance to claim the belt held for the past 11 months by Cruz (22-2), who ended up with a gaping cut over his left eye. Garbrandt used precise striking, multiple takedowns and remarkable charisma to win over the judges, who all favored him by scores of 48-46, 48-46 and 48-47.
Cruz hadn't lost an MMA fight since March 24, 2007, winning 13 straight bouts in a career marked by multiple comebacks from major injuries.
The 25-year-old Garbrandt is a native of Uhrichsville, Ohio, who trains in the Sacramento-area gym of veteran fighter Urijah Faber.
Cruz and Garbrandt taunted each other repeatedly during the promotion of their bout, and their genuine dislike translated into a frenzied five-round fight. Garbrandt lived up to his boundless promise with a resourceful effort against Cruz, who was favored to defend his belt.
Instead, Garbrandt landed more shots and did more damage from the opening round. Both fighters landed significant strikes in the first two rounds, but Garbrandt turned the fight in his favor early in the third round during the exchange that opened the cut on Cruz's face.
Garbrandt put on a performance in the fourth round, repeatedly knocking down Cruz when he wasn't dancing away from the champion. The unfriendly rivals congratulated each other after the bout, but Cruz didn't appear surprised when Garbrandt's hand was raised.
After receiving his belt, Garbrandt wrapped it around the waist of Maddux Maple, a 10-year-old leukemia survivor from northern Ohio who has been his motivational talisman.
Garbrandt's first title defense could be against T.J. Dillashaw, who dominated John Lineker in a unanimous-decision victory moments earlier at UFC 207, winning 30-26 on all three cards. Dillashaw (15-3) showed off precise striking and a superior ground game to trounce the Brazilian brawler.