UFC 100 was a mixed martial arts event held by the Ultimate Fighting Championship (UFC) on July 11, 2009 in Las Vegas, Nevada.
The event included two championship bouts (Heavyweight and Welterweight) and a fight between the two coaches of the most recent season of The Ultimate Fighter reality series on Spike TV. UFC 100 is reportedly the highest grossing pay-per-view event to date for the UFC, with 1.6 million buys. Along with UFC 83, UFC 100 holds the highest gate revenue record for a UFC event, tied with 5.1 million, with tickets sold-out before they were made available to the general public.
The main event featured a rematch between UFC Heavyweight Champion, Brock Lesnar, and UFC Interim-Heavyweight Champion, Frank Mir. This title unification bout occurred as a result of former UFC Heavyweight Champion, Randy Couture's year-long "resignation" from the company. During his absence, the UFC created an Interim-Heavyweight Championship that Antonio Rodrigo Nogueira had won by defeating Tim Sylvia. Nogueira and Mir were opposing coaches in The Ultimate Fighter: Team Nogueira vs Team Mir, and would fight for the interim championship at UFC 92.
During the airing of the television program, Couture returned to the company and UFC President Dana White announced that he would defend his title against Lesnar at UFC 91. This meant that the winner of the Couture-Lesnar and Nogueira-Mir matches would face off for the undisputed crown. In his fourth professional fight, Lesnar defeated Couture via technical knockout to become the UFC Heavyweight Champion. Mir was crowned the interim champion after becoming the first fighter to ever finish Nogueira. The match between Lesnar and Mir was set for UFC 98. However, Mir suffered a knee injury during training and was forced out of the bout. It was announced a day later that the proposed fight would be moved to UFC 100 instead. Lesnar and Mir had fought once before, during Brock's UFC debut at UFC 81; which Mir won via a kneebar submission.
The co-main event featured Georges St-Pierre, defending his Welterweight Championship against Thiago Alves. Alves entered the fight with consecutive victories over three of the top UFC welterweights in Josh Koscheck, Matt Hughes and Karo Parisyan. Meanwhile, St-Pierre entered the bout with successful title defenses against top contender Jon Fitch and UFC Lightweight Champion BJ Penn.
The third match on the main card was between top middleweight contenders, Dan Henderson and Michael Bisping. The two served as opposing coaches on the ninth season of The Ultimate Fighter, The Ultimate Fighter: United States vs. United Kingdom, with Henderson leading Team USA and Bisping heading Team UK. Bisping has compiled a professional MMA record of 18–1, and has been undefeated since moving to middleweight. He entered the bout with victories over Charles McCarthy, Jason Day and Chris Leben. On June 2, it was announced that the winner of the Henderson-Bisping Fight will get a shot at the UFC Middleweight Championship against Anderson Silva at a later date.
A preliminary match-up between Dong Hyun Kim and Jonathan Goulet was originally scheduled to be on the card, but Goulet had to pull out of the bout with a shoulder injury. TJ Grant was named as his replacement.