Guy Mezger (born January 1, 1968) is an American martial artist, who competed in professional combat sports including sport karate(he is in sport karate museum hall of fame) full contact karate, kickboxing, and boxing, but is most recognized as a mixed martial arts (MMA) fighter. Mezger retired from professional competition on January 25, 2005. He is associated with Lion's Den and runs their school in Dallas. Mezger was a champion in mixed martial arts in two different promotions, the UFC and Pancrase. He holds wins over Tito Ortiz, Masakatsu Funaki, Yuki Kondo, Semmy Schilt, and Minoru Suzuki.

Mezger has trained with many great martial arts competitors and trainer/instructors; his main trainers have been Vince Tamura (judo), Willie Thompson (wrestling), Billy "Jack" Jackson (kickboxing), and Ken Shamrock (shootfighting). Mezger has co-written a book, The Complete Idiot's Guide to Kickboxing; he had an uncredited speaking role in the first-season episode of Walker, Texas Ranger titled "Night of the Gladiator".

Born in Houston and raised in Dallas, Mezger wrestled in high school and also practiced taekwondo, in which he holds a 7th degree black belt. As a professional kickboxer, he won the U.S. Heavyweight title before winning the WKC World Heavyweight Championship in June 1995; a title he defended once before retiring from the sport to compete in Pancrase.

Mezger began his mixed martial arts career in the Ultimate Fighting Championship at UFC 4 in an alternate match against Jason Fairn. Before the fight, Mezger had asked Fairn to make a 'gentlemen's agreement' not to pull hair due to the fact both men had long hair. Mezger won the bout by TKO after landing a multitude of strikes from full mount. Mezger next fought at UFC 5 in an alternate match against John Dowdy, defeating him by TKO in little over two minutes by mounted strikes. Mezger began to train with Ken Shamrock and became a member of Ken's submission fighting team, the Lion's Den, and joined the Pancrase organization in Japan.

In his second match at the Pancrase 1995 Anniversary Show, Mezger faced Pancrase co-founder and Shamrock's trainer Masakatsu Funaki. The American fighter performed well, scoring high kicks and forcing Funaki to pull guard and work from the bottom, but he was eventually caught in a leglock exchange from his own trip and submitted with an Achilles lock.

In December 1995, Mezger was involved in a controversial match against the other Pancrase co-founder, lauded wrestler Minoru Suzuki. Mezger controlled the match, but he landed an accidental kick to the groin at 7:15 in the first round and was disqualified by the referee. His corner protested and defended the accidental nature of the strike, but the result was not changed. Guy's personal record shows it as a no contest.

Nonetheless, Mezger accumulated a Pancrase 16–7–2 record and become the 7th ever King of Pancrase world champion with a win over Masakatsu Funaki.

Mezger would not return to the UFC until UFC 13 where he competed in the Lightweight Tournament (200 lbs & under). In his first bout, Mezger fought top ranked judo fighter, Christophe Leininger. Leininger was able to score a takedown and gain mount at one point, but he was otherwise soundly beaten as Mezger battled his way to a decision victory. Mezger broke his hand during this fight but continued in the tournament.