Sean Keith Sherk (born August 5, 1973) is an American retired mixed martial artist and former UFC Lightweight Champion. Sherk competed in the Ultimate Fighting Championship and was one of the first combatants to have been a championship competitor in multiple weight divisions (having also competed for the UFC Welterweight Championship). He was the second UFC Lightweight Champion in the organization's history after Jens Pulver vacated his title five years earlier. Sherk also spent time competing in the Japan-based organizations, PRIDE Fighting Championships and Pancrase; going undefeated in both promotions. He holds one of the longest undefeated streaks in mixed martial arts history, with only four career losses, all to fellow-UFC Champions. Sherk announced his official retirement from mixed martial arts competition in September 2013 having last fought three years prior.

Recognized for his role in the resurgence of the UFC Lightweight division, Sherk became the first UFC Lightweight Champion since the division was re-introduced by the UFC in 2006. He successfully defended the UFC Lightweight Championship against Hermes Franca at UFC 73, after winning the title in a Fight of the Night award winning performance against Kenny Florian at UFC 64. Sherk is noted to have taken part in some of the most important fights in the UFC's lightweight division, where he is considered to be one of the greatest lightweight competitors of his era.

Sherk was born on August 5, 1973, in St. Francis, Minnesota. Sherk began Wrestling and Weightlifting at age seven. He continued to wrestle for eleven years, and in that time he wrestled over 400 matches. After finding Greg Nelson's Minnesota Martial Arts Academy in 1994, Sherk began to practice Judo, Boxing, Shootfighting, and Muay Thai. In 1999, Sherk defeated Roscoe Ostyn by decision in his first MMA fight.

Sherk began his mixed martial arts career in 1999, where he went on an eight-fight win streak before signing with the Ultimate Fighting Championship (UFC) in 2001. Sherk fought only once before leaving the UFC. He returned in 2002 and fought Matt Hughes for the UFC Welterweight Championship in 2003. He lost the match and went on to sign with Pride Fighting Championships (Pride).

Sherk made his UFC debut at UFC 30, where he defeated Tiki Ghosn. After going 5–0–1 outside the UFC, he was brought back to fight Jutaro Nakao at UFC 36, a fight which Sherk won via unanimous decision. On September 27, 2002, at UFC 39, Sherk defeated Benji Radach when the fight was stopped due to a cut Radach had received.

Sherk was then chosen to fight Matt Hughes for the UFC Welterweight Championship on April 25, 2003. Sherk lost the fight via unanimous decision after going five five-minute rounds. In a back and forth battle, Sherk won two of the overall five rounds, becoming the first and only fighter to ever fight a full five round fight with then-champion Matt Hughes.

Following his loss to Hughes, Sherk fought three more times in 2003, winning all three bouts. In 2004, he signed with Pride and made his debut with the Japanese-based organization at Pride Bushido 2. Sherk defeated Ryuki Ueyama via unanimous decision, in what was his only outing in the company.

According to Sherk's website, the Japanese fans are the ones responsible for giving him the nickname "The Muscle Shark." While he was popular in Japan, Sherk found it difficult to travel and support his family and found he could make a better living fighting as a main event on local shows. He also cited his lack of health insurance for making this decision.^{[citation needed]}