UFC 100 was a mixed martial arts event to 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 Ultimate Fighter. Also featured will be the UFC debut of Yoshihiro Akiyama. The event was announced as sold-out, before the tickets were even 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. Nogeuira and Mir were scheduled to be opposing coaches in The Ultimate Fighter 8: Team Nogueira vs. Team Mir, and would fight for the interim championship at the conclusion of the show. The Ultimate Fighter is a single-elimination reality series featuring fledging professional MMA fighters competing for the title of The Ultimate Fighter, and a six-figure, multi-fight contract with the UFC.
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 just 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 knockout Nogueira. The match between Lesnar and Mir was now 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. Lesnar controlled much of the bout, and mounted a successful ground and pound attack. During the exchange on the ground, Mir got hold of Lesnar's leg and locked up a Kneebar. Lesnar could not escape and was forced to tap out, awarding Mir the victory.
The co-main event featured Yahoo! Sports' top pound-for-pound fighter (tied with Fedor Emelianenko), Georges St. Pierre, defending his UFC Welterweight Championship against MMAWeekly.com's 2008 Fighter of the Year, Thiago Alves. St-Pierre is a fighter with strong Wrestling, and trains with the Canadian Olympic team. His conditioning, strength and athleticism have also been identified as strong attributes. Thiago Alves is well-versed in the Muay Thai striking discipline, and was dubbed by Setanta Sports as the "Muay Thai wrecking ball". Alves enters the fight with consecutive victories over three of the top UFC Welterweights in Josh Koscheck, Matt Hughes, and Karo Parisyan. Meanwhile, St. Pierre enters the bout with successful title defenses against top contender Jon Fitch, and UFC Lightweight Champion BJ Penn.
The third match on the card occured between two top Middleweight contenders, Dan Henderson and Michael Bisping. The two were opposing coaches in the ongoing season of The Ultimate Fighter 9: United States vs. United Kingdom, with Henderson leading Team USA and Bisping heading Team UK, respectively. Henderson is a former Olympic Greco-Roman wrestler also noted for the punching power in his right hand. He is the first fighter to concurrently hold championship belts in two separate weight classes in a major organization (he did so in PRIDE FC). Following the sale of Pride FC, Henderson entered the UFC and lost his championships to Quinton Jackson and Anderson Silva in back-to-back unification contests. His latest fight was a decision victory over Rich Franklin at UFC 93. Bisping started his career as a Light Heavyweight and was the winner of the third season of The Ultimate Fighter. He compiled a professional MMA record of 18–1 before entering the fight with Henderson, and had been undefeated since moving to Middleweight. He entered the bout with victories over Charles McCarthy, Jason Day and Chris Leben.
 Official Fight Card
 Main Card
- Heavyweight Championship bout: Champion Brock Lesnar vs. Interim Champion Frank Mir
- Welterweight Championship bout: Georges St. Pierre vs. Thiago Alves
- Welterweight bout: Jon Fitch vs. Paulo Thiago
- Middleweight bout: Dan Henderson vs. Michael Bisping
- Middleweight bout: Yoshihiro Akiyama vs. Alan Belcher
 Preliminary Card
- Light Heavyweight bout: Mark Coleman vs. Stephan Bonnar
- Lightweight bout: Mac Danzig vs. Jim Miller
- Light Heavyweight bout: Jon Jones vs. Jake O'Brien
- Welterweight bout: Dong Hyun Kim vs. T.J. Grant (Grant replaced Jonathan Goulet)
- Middleweight bout: C.B. Dollaway vs. Tom Lawlor
- Lightweight bout: Matt Grice vs. Shannon Gugerty
|Lighweight||Shannon Gugerty||Matt Grice||Submission (Guillotine Choke)||1||2:36|
|Middleweight||Tom Lawlor||CB Dollaway||Submission (Guillotine Choke)||1||0:55||Lawlor won Submission of the Night Honors|
|Welterweight||Dong Hyun Kim||T.J. Grant||Decision (Unanimous)||3||5:00|
|Light Heavyweight||Jon Jones||Jake O'Brien||Submission (Guillotine Choke)||2||2:43|
|Lightweight||Jim Miller||Mac Danzig||Decision (Unanimous)||3||5:00|
|Light Heavyweight||Mark Coleman||Stephan Bonnar||Decision (Unanimous)||3||5:00|
|Middleweight||Yoshihiro Akiyama||Alan Belcher||Decision (Split)||3||5:00||Fight won Fight of the Night Honors|
|Middleweight||Dan Henderson||Michael Bisping||KO (Punch)||2||3:20||Henderson won Knockout of the Night Honors|
|Welterweight||Georges St. Pierre||Thiago Alves||Decision (Unanimous)||5||5:00||St. Pierre defended UFC Welterweight Championship|
|Heavyweight||Brock Lesnar||Frank Mir||TKO (Punches)||2||1:48||Lesnar defended UFC Heavyweight Championship; Unified Titles|
|Welterweight||Jon Fitch||Paulo Thiago||Decision (Unanimous)||3||5:00|
Fighters were awarded $100,000 bonuses.