In the rematch, Nunes did exactly what she promised. With a new southpaw stance on the feet and a renewed wrestling game, Nunes defeated Peña via a dominant unanimous decision (50-45, 50-44, 50-43) in the main event of UFC 277 on Saturday night at American Airlines Center.
With the victory, Nunes, the greatest women's fighter in MMA history, reclaimed the UFC women's bantamweight title she lost to Peña at UFC 269 in December.
"The lioness, if they don't get the prey the first time, I set the trap and I know I'd get it the second time," Nunes said in her postfight interview.
Nunes said this fight was all about strategy and that the key was her going southpaw. Then, when Peña got comfortable on the feet, Nunes said she started to go for the takedowns.
"The main thing was my southpaw," Nunes said. "I know I was going to catch her with that tonight. I knew she was not going to be able to adjust to that. She's not a striker. ... She was so confused. I'm not very good yet with my southpaw. I told my coach, 'I'm going to see how I feel.' I tried tonight, and I felt good."
Nunes dropped Peña with a check right hook out of the southpaw stance several times in the early rounds. As the fight wore on, Nunes began using her wrestling to take Peña down over and over then cut her up from top position with slicing elbows.
Peña had several bad cuts on her face after the fourth round. Nunes nearly had a rear-naked choke finish in the fifth, but Peña was incredibly tough and fought it off.
Nunes had moments where she might have been able to finish the fight but perhaps didn't want to make the mistake she did in the first fight, where she got tired after trying to finish Peña in the first round.
"Julianna is tough as nails," UFC president Dana White said. "Her will to win is second to none. She wanted to win. As dominant as Amanda was, and she was dominant tonight -- I don't think it was close in any way, shape or form; I thought it was a complete shutout -- she still looked a little gun-shy to me.
"She had Julianna hurt many times, had her on crazy legs many times and never really went in for the kill."
After the first fight, Nunes left her longtime gym, American Top Team, and moved into a private facility she has dubbed Lioness Studio. She trained under coach Roger Krahl, who was in her corner for some of her biggest fights, like the knockout of Ronda Rousey.
"The best thing I did was make my gym," Nunes said. "In my gym, I feel like I'm safe. I feel like I can grow and evolve."
In the second round, Nunes knocked Peña down three times, setting the record for most knockdowns in a round ever among women in UFC, according to ESPN Stats & Information research. Overall, Nunes outlanded Peña 85-60 in significant strikes and took Peña down six times in eight attempts.
Nunes' 15 UFC wins is the most among women in promotion history. This was her 10th title fight win, the most among women in UFC history and fifth best overall. Nunes is the first woman in UFC to regain the bantamweight title after losing it.
Saturday's fight was the 13th immediate rematch ever in UFC after one fighter took the belt from another. Saturday marked just the third time in those instances that a fighter reclaimed the title. Randy Couture did it against Vitor Belfort in 2004, and Deiveson Figueiredo did it in January over Brandon Moreno.
Judges Douglas Crosby and Sal D'Amato each scored the second round 10-8 for Nunes, while Crosby also scored the fifth 10-8 for Nunes. The other judge, Jacob Montalvo, did not score a 10-8 in any round.
Coming in, ESPN had Nunes ranked No. 2 and Peña ranked No. 4 on its pound-for-pound women's MMA list. At bantamweight, Peña was No. 1 and Nunes was No. 2.
Nunes (22-5) was on a 12-fight winning streak before losing to Peña, with wins during that stretch over the likes of Rousey, Cris Cyborg, Miesha Tate and Holly Holm. The Brazilian slugger, who trains out of South Florida, is also the UFC women's featherweight champion.
Nunes, 34, is the first UFC fighter to defend two titles in separate weight classes concurrently, and she has seven title defenses across two divisions, the most for a woman in UFC history.
Asked in the postfight news conference what would be next for Nunes, White said a third fight with current UFC women's flyweight champion Valentina Shevchenko was "not a bad idea."
Peña (12-5) had won two straight and seven of her nine previous fights overall coming in. The fighter, who was born in Washington state and trains out of Chicago, beat Nunes via second-round submission seven months ago as a heavy underdog. Peña, 32, was the first women's Ultimate Fighter tournament winner in 2013 and had been calling for a title shot for years before actually getting one.
She was transported to a hospital after the bout, and White said Peña was going to see a plastic surgeon to deal with the cuts Nunes caused on her face.
"Julianna has a big chunk missing from her forehead," White said, adding that Peña "got pretty banged up tonight."
Before dropping the bantamweight title to Peña, Nunes had beaten every fighter who ever held the UFC women's bantamweight or featherweight belts. She completed that feat again with Saturday's win.
"Tonight, it was proven that was a lucky day for her," Nunes said of Peña's title win in December. "If she needed to become a champion like that, she had her time, and now it's over."