A high schooljudoka, Yamamoto made his professional debut on the then puroresu promotion Fighting Network RINGS in 1991 against Masayuki Naruse, who was also making his debut. When the company turned to mixed martial arts, he had his first fight on January 25, 1995, facing off another debutant, Dutch fighter Hans Nijman. Yamamoto lost the bout when he was knocked out only 43 seconds into the fight. After winning his next bout for via unanimous decision at an event in Amsterdam for the organization, Yamamoto faced off against legendary Brazilian jiu-jitsu specialist, Rickson Gracie at Vale Tudo Japan 1995. Yamamoto gave the undefeated fighter arguably the most difficult fight of his career, but the former professional wrestler ultimately lost via rear-naked choke at 3:49 of the third round, the referee having stepped in when Yamamoto refused to tap out. This would be the only fight that Gracie ever had that didn't end with him winning in the first round. The fight with Gracie elevated Yamamoto's status. In his next bout, he won via neck crank submission before losing via keylock submission to his teacher Akira Maeda and then to Ricardo Morais, who knocked out Yamamoto 46 seconds into the fight. In his next bout, Yamamoto faced David Khakhaleishvili, an Olympic Gold Medalist in judo. Yamamoto defeated the Georgia fighter via TKO 40 seconds into the fight before competing in the. Yamamoto won his fight in the quarter-finals of the tournament and then lost in his semi-finals match against Kiyoshi Tamura. Yamamoto then went 7-7-1 in his next 15 fights with wins over Valentijn Overeem, Joop Kasteel, and Tsuyoshi Kosaka before being invited to compete in PRIDE.
PRIDE
Yamamoto made his PRIDE debut at Pride 16 on September 24, 2001 against Brazilian Assuerio Silva and was dominated, losing via only 11 seconds into the fight via TKO. However, the former professional wrestler would bounce back, defeating South AfricankickboxerJan Nortje via armbar submission less than two minutes into the fight. In his next bouts he was knocked out by former NFLlinemanBob Sapp and then lost via unanimous decision to Guy Mezger before picking up his next win against Alexander Otsuka after Otsuka injured his leg in the second round. In his next bout at Pride Final Conflict 2003 Yamamoto fought Heath Herring and performed well against the Sambo and Muay Thai specialist but ultimately lost via rear-naked choke submission in the third round. In his next bout the 13-16-1 Yamamoto faced two-time UFC Heavyweight Tournament Champion, Mark Kerr. Yamamoto pulled off a huge upset win, defeating the former All-American via TKO only 40 seconds into the fight. Yamamoto's next appearance for the organization was at Pride Bushido 2 against future 2006 Pride World Grand Prix ChampionMirko Cro Cop. Yamamoto lost the bout via TKO 2:12 into the first round. Yamamoto fought once more the organization at Pride Bushido 3 against Choi Mu-Bae and lost via unanimous decision.
Post-PRIDE
After PRIDE, Yamamoto fought for K-1 Hero's, and DEEP, but has not won since his TKO win over Mark Kerr, losing his last nine fights.
