Rousey was shockingly knocked out by challenger Holly Holm, who dominated for a second-round victory at UFC 193.
"I've never spent this much time in the gym than I have for any fight," said Holm, a 34-year-old JacksonWink MMA fighter. "It really all came together."
The fight-ending kick, which many weren't expecting, came 57 seconds into the second round.
"I just felt like, 'How could I not do this? I had the best coaching from wrestling to grappling to stand-up. Everything we worked on presented itself in the fight," an ecstatic Holm said as a UFC record crowd of 56,214 fans at Etihad Stadium cheered.
"I'm trying to take it in, but it's crazy," she added.
The shocking loss ended Rousey's impressive run of 12 victories, some of which occurred within seconds of entering the ring.
Holm and Rousey won fight of the night, each earning a $50,000 bonus. However, Holm won another $50,000 for her performance of the night bonus.
Watch the brutal head-kick in the video above.