Mamoru Yamaguchi is a Japanese mixed martial artist competing in the flyweight division. He is a former Shooto Bantamweight Champion and a former Shooto Featherweight Champion. He has been consistently ranked as one of the top flyweights in the world and is known for his counter striking and his trilogy with former Shooto Bantamweight Champion Shinichi Kojima. His afro is a tribute to Japanese boxing legend Yoko Gushiken.

Yamaguchi became the first-ever Shooto Featherweight (132 lbs.) Champion by defeating Jin Akimoto at Shooto - R.E.A.D. Final via unanimous decision. Yamaguchi's first title defense came against Masahiro Oishi at Shooto - To The Top 10 where Yamaguchi lost via submission in the first round.

On December 14, 2003 Yamaguchi met Yasuhiro Urushitani at Shooto - Year End Show 2003 for the first-ever Shooto Bantamweight (123 lbs.) Championship. Yamaguchi defeated Urushitani via unaimous decision to claim the title. He defended the title twice afterwards, first against Robson Moura then against Shinichi Kojima, both fights ending in draws. His third defense came against Shiniji Kojima, who defeated Yamaguchi via submission in the first round.

After going 3-1 in his next four fights under Shooto, Yamaguchi fought Kojima for a third time and this time as a challenger for the Shooto Bantamweight Championship he had lost. Yamaguchi was unsuccessful again against Kojima, losing via submission in the third round. Yamaguchi next fought for the vacant KOTC Juniior Flyweight (125 lbs.) Championship against Frank Baca. Yamaguchi defeated Baca via second round submission and claimed the title.