Amanda Nunes bludgeoned Raquel Pennington for more than 20 bloody minutes on her way to retaining the 135-pound strap.
Nunes returned for the first time since a split decision win against Valentina Shevchenko in September last year and showed no signs of losing her status as the best female bantamweight in the world.
She butchered Pennington with leg kicks before appearing to break her nose with a knee in the clinch.
The American, who was fighting for the first time since defeating Miesha Tate in late 2016, told her corner she was done after round four but was sent back out for more punishment.
"She was physically in the fight in round five but she wasn't mentally in the fight," a UFC commentator said.
"We could see, it was right in front of us, her nose basically exploded."
Pennington's corner faced immediate criticism for telling her to "change her mindset" instead of throwing in the towel.