Shooto

After honing his submission skills under Satoru Sayama, Sato made his professional debut in MMA on December 7, 1994, winning over Michael McAuliffe with a calf slicer, the first time the move had been used in mixed martial arts. He ascended fastly in the roster by winning 10 straight matches, becoming a fan favourite for his aggressive and spectacular style of grappling.^[3] As soon as his fourth match, he submitted his opponent with a flying inverted triangle choke, and at his seventh, he gained worldwide fame by submitting the Brazilian jiu-jitsu black belt Ricardo Botelho, taking advantage of his guard usage to attack him with leglocks and submit him with a heel hook. Sato also defeated another big name of the art when he beat John Lewis, who he submitted in revenge for a match at the Vale Tudo Japan which ended in a draw. Sato would become a usual fighter for this event, representing Shooto in six out of the nine years it had place.

Sato's winning streak finally broke in 1998 when he lost a highly publicized match to Canadian Jiu-Jitsu Champion Joel Gerson by a surprising armbar in the first round of a non-title event in 1998. Sato avenged his defeat in the next Canadian Jiu-Jitsu open tournament the same year, where he faced Gerson again at the finals and submitted him with an ankle lock. After returning to Japan, Sato got also which is thought to be the fastest submission victory in MMA history when he submitted Charles Diaz in a mere five seconds with a flying armbar.^[4]

At Vale Tudo Japan 1998, Sato went against André Pederneiras of the Nova União in a vale tudo rules match. Sato suffered the first takedown, but he almost caught Pederneiras with a persistent armbar. The two exchanged minor strikes in Sato's guard before the Brazilian grappler decided to stand-up, and the match seemed to go stall from their respective positions. However, suddenly Pederneiras sidestepped Sato and landed a soccer kick to the head, following with heavy punches to the face for the doctor stoppage.

On May 29, 1999, he would fight former trainee Caol Uno for the vacated Shooto Welterweight Championship. Sato captured Uno's back mere two minutes into the match and pursued the rear naked choke, but despite threatening to lock the hold at several points, Uno managed to escape free. Going into the second round, Sato controlled the stand-up, opening a cut on the right side of his opponent's face and scoring a takedown, but he found himself having to defend from his guard while Uno capitalized to land ground and pound. At the third round, Uno started controlling the wrestling game and solidifying his assault to Sato's guard, and ultimately, when a clearly tired Rumina attempted a takedown, Caol took his back and locked a rear naked choke for the tap out.

Sato returned to Vale Tudo Japan in 1999 fighting Rafael Cordeiro, founder and instructor for the famed Chute Boxe team. The match was short, as Rumina scored a belly to belly suplex and right after locked a kneebar, making Cordeiro tap out.

Rumina rematched Uno in December 2000 for the title. In a cautious and tentative affair, Sato captured the back of his opponent while standing and attempted to lock a choke while Uno defended with a Kimura lock threat, but none was successful. However, when Sato finally released him, Uno capitalized on his bad position and landed a knee strike to the face and a right punch to the temple, knocking Sato out.

After his second defeat to Uno, Sato faced Marcio Ramos Barbosa of Barbosa Jiu-Jitsu, who had submitted Caol before the rematch. Sato won the fight by unanimous decision.

In December 2001, Sato faced rising star Takanori Gomi for the vacated Shooto World Lightweight Championship. Playing clinch to avoid his foe's dangerous striking, Rumina led the fight to the mat and besieged Gomi with submissions attempts, including armbars both from and against the guard, toeholds from inverted positions and an omoplata that almost finished the fight, but Takanori was consistently able of getting out of them. The second round saw Gomi landing damaging punches, as well as Sato utilizing an active guard to counterattack, and the striker eventually bypassing his defenses and controlling the pace. Come the third and final, Gomi landed the last punches against the guard before winning the bout by unanimous decision.

Despite PRIDE Fighting Championships and Ultimate Fighting Championship being both interested in signing him up, Rumina famously vowed to never compete in any other organization than Shooto (unless representing Shooto in special matches) until he won a title there.^[3] He won the Shooto Pacific Rim Lightweight Tournament on March 11, 2005, from Makoto Ishikawa, but still remained in the promotion until his retirement in May 2014.

On July 10, 2009, Sato competed in a grappling super fight at the UFC 100 Fan Expo Grappler's Quest against Ulysses Gomez, winning by submission with an inverted triangle choke/wrist lock combination.

Mixed martial arts exhibition record