Stephan Patrick Bonnar (April 4, 1977 – December 22, 2022) was an American mixed martial artist and professional wrestler. Bonnar competed as a Light Heavyweight in the UFC for most of his career. Bonnar was the runner-up on The Ultimate Fighter 1; his TUF Ultimate Finale loss to Forrest Griffin is widely considered to be the most important fight in the history of the UFC.

Bonnar was born in Hammond, Indiana, and raised in Munster, Indiana, attending Munster High School. Bonnar had an extensive background in combat sports, beginning with wrestling when he was 10 years old, Taekwondo at age 12, Brazilian jiu-jitsu at age 22, and then added boxing and Muay Thai when he was 24 years old. He earned his black belt in Taekwondo at the age of 16 and was a two-time Golden Gloves Champion in the super heavyweight division. He attended and graduated from Purdue University in 2000, where he competed in wrestling, and earned a degree in sports medicine.

Bonnar began training Brazilian jiu-jitsu with Carlson Gracie during the summer of 1999, under whom he received his purple belt. Bonnar also trained in Muay Thai during several trips to Thailand. Since the beginning of 2010 Bonnar trained in Muay Thai in Las Vegas under Master One Kick Nick Blomgren at One Kick's Gym.

Due to his strong performance in the Light Heavyweight finals, where he lost a unanimous decision to Forrest Griffin in a back and forth fight, Bonnar was awarded a six-figure UFC contract along with Griffin. After his loss to Griffin, Bonnar became the staple of the regular UFC Ultimate Fight Night shows at the Hard Rock Hotel and Casino in Las Vegas, defeating such notables as Sam Hoger, James Irvin and Keith Jardine before succumbing to Rashad Evans by majority decision. In 2006 Bonnar lost a unanimous decision against Forrest Griffin in the long-anticipated rematch at UFC 62. After UFC 62, Bonnar tested positive for the banned substance Boldenone, a type of anabolic steroid. The commission issued a nine-month suspension on Bonnar's fighter's license and fined him $5,000.

Bonnar was forced to withdraw from his fight against Matt Hamill at UFC Fight Night 13 due to a serious knee injury he suffered during training. He returned to action at UFC 94 against Jon Jones, which resulted in a loss via decision. In his next fight at UFC 100 Bonnar lost a unanimous decision to Mark Coleman.

Bonnar mentioned on the MMA Live Post Fight Show for UFC 101 that he was considering a drop down to Middleweight after two disappointing losses at Light Heavyweight. Despite this, Bonnar next faced Krzysztof Soszynski on February 21, 2010, at UFC 110. Soszynski was victorious at 1:04 in the third round, due to TKO (Cut). Video replays that showed that the cut was opened up by a clash of heads led Bonnar to appeal referee John Sharp's decision. It was announced on March 10 that he had lost the appeal and the result would stand as a TKO win for Soszynski.

At UFC 116, Bonnar won the rematch with a second-round TKO against Soszynski after catching him with a knee. The win broke Bonnar's three fight losing streak, bringing his UFC record to 6-6. The fight earned Bonnar and Soszynski Fight of the Night honors alongside Yoshihiro Akiyama and Chris Leben's fight.

Bonnar fought Igor Pokrajac on December 4, 2010, at The Ultimate Fighter 12 Finale. He won the bout via unanimous decision (29–26, 29–26, and 29–26).