Overeem landed a kick to the liver that knocked Lesnar down, then swarmed on him and landed punches on the ground until referee Mario Yamasaki stepped in to stop the fight. The bout lasted only two minutes, 26 seconds.
Afterward Lesnar, who has been battling diverticulitis over the last two years, said he would no longer compete in the UFC.
"My hat's off to Alistair Overeem," Lesnar said afterward. "I've had a difficult couple of years and I'm going to officially say tonight was the last time you're going to see me in the Octagon."
A former WWE star, Lesnar has enough money that he never has to work again, and it's no surprise that he doesn't feel the need to put his body through the rigors of an MMA career anymore.
As for Overeem, he was as good as advertised.
"Today was all about bad intentions -- first or second round, I promised," Overeem said afterward, referring to his pre-fight prediction that he would finish Lesnar in one or two rounds.
Lesnar, surprisingly, opened the fight by throwing a couple of leg kicks, and Overeem responded by throwing a looping right hand that missed. A minute into the first round Lesnar attempted a single-leg takedown, and Overeem shook him off. Overeem went on the attack and landed knees to Lesnar's body, while Lesnar landed a punch that opened a cut over Overeem's right eye.
But those attacks to Lesnar's body were all Overeem needed. Lesnar's striking defense just isn't good enough to handle the striking of a former K-1 champion like Overeem, who referred to his finishing shot as a "K-1 body kick.
Overeem, a former champion in Strikeforce, Dream and K-1, will now face UFC heavyweight champion Junior dos Santos. That fight will determine who's at the top of a UFC heavyweight division that no longer includes Lesnar.