Rousey (9-0) successfully defended the UFC bantamweight title for the third time at UFC 170 on Saturday, finishing McMann via TKO just 1:06 into the first round.
After trapping McMann (7-1) against the fence, Rousey scored the first knockout of her career with a hard knee to her opponent's midsection. Referee Herb Dean stepped in immediately, as McMann doubled over from the shot.
McMann appeared to recover quickly once the fight was called, prompting a large portion of the Mandalay Bay Events Center crowd to boo the stoppage. McMann, however, didn't fight the outcome.
"I thought it was a good fight," McMann said. "I got hit in the liver. No matter how hard you train, you can't make your liver stronger."
It marks the first knockout of Rousey's mixed martial arts career. She had finished all eight of her previous opponents via armbar submission. It is the eighth time Rousey has finished a fight inside the first round.
The headlining bout was billed as a matchup between two world-class grapplers. Rousey won a bronze medal in judo for the U.S. in the 2008 Olympics. McMann was a silver medalist in freestyle wrestling for the U.S. in 2004.
It was Rousey's striking that stole the show, however, as she aggressively pushed McMann to the fence from the opening bell.
McMann answered with counter right hands, but couldn't find a way to circle off the fence. She managed to back Rousey off briefly at one point with a clean straight right, but Rousey was back on her in moments.
The win set a UFC record for briefest time span between title defenses.
Rousey defended the belt at UFC 168 on Dec. 28. The 56-day turnaround equals one made by former welterweight champion Matt Hughes in 2006, although he lost the title to Georges St-Pierre in the second fight.