Early career After winning the gold medal, Ishii expressed his wishes to apply his talents in the sport of mixed martial arts (MMA) to the dismay of the Japanese Judo Federation. Ishii originally confirmed that he signed a contract with Japanese MMA promotion
Fighting and Entertainment Group (FEG) and was rumored to debut in early 2009 in their fight series,
DREAM. Japanese newspaper
Sports Nippon then reported that Ishii would make his ring debut on December 31, 2009, at the annual
K-1 Premium Dynamite!! event at the
Saitama Super Arena. However, Ishii announced that he rejected FEG's offer, and was planning to pursue his MMA career in the US'
Ultimate Fighting Championship (UFC). He headed to Florida where he trained with one of MMA's top camps
American Top Team. On June 1, 2009, Ishii announced he would join Japanese MMA organization
World Victory Road. He officially signed with the organization at a public signing event on June 4, 2009. On September 14, 2009, it was announced that Ishii would make his MMA debut against fellow Olympian
judo gold medalist
Hidehiko Yoshida. Although the fight was originally to take place at
World Victory Road's New Year show on December 31, when that event was cancelled, the fight was moved to DREAM's
Dynamite!! 2009 card on New Year's Eve as a co-promoted show. Ishii, who was a large favorite in the fight, was dominated in the first round by the more-experienced Yoshida, taking many strikes including a right overhand that staggered the newcomer, along with a series of knees and
uppercuts from the clinch. Ishii, however, came back with a more competitive second round, but took a point deduction after illegally kneeing Yoshida in the groin. Ishii continued to find his range in the third round with punches, but ultimately lost via unanimous decision. For his second fight, Ishii faced New Zealand heavyweight
kickboxing champion Tafa "Thumper" Misipati on May 15, 2010, in
Auckland, New Zealand. The fight was the main event at the ETK production
X-plosion: New Zealand vs. Japan. Ishii won via
submission (
armbar) in the first round. In June 2010, Ishii caused controversy during an MMA exhibition match on the undercard of
X-1 World Events: Nations Collide in Hawaii. After knocking down his opponent Myles Tynanes at the end of the first-round, Ishii failed to stop punching his downed foe despite the referee's attempts to halt the action. He was subsequently disqualified for his actions. On September 25, 2010, Ishii fought
Ikuhisa Minowa, a
shoot wrestler known for facing much larger opponents, at
Dream.16 and won via unanimous decision. Ishii's next MMA fight was supposed to be against
Antz Nansen at the
K-1 World MAX 2010. However, his opponent was changed to
Katsuyori Shibata at the last minute. Ishii won via Kimura in the first round. Ishii continued to train at
Black House in Brazil with notable fighters such as
Lyoto Machida and
Anderson Silva. Prior to Silva's fight with
Chael Sonnen Ishii hurt Silva's ribs. Since then he has trained at
Mark Muñoz's gym Reign MMA, along with
Mayhem Miller,
Fabrício Werdum and other notable fighters. Ishii then fought French kickboxing star
Jérôme Le Banner in a mixed martial arts bout at the
Dynamite!! 2010 New Year's Eve event. He won the fight via unanimous decision. Ishii was scheduled to make his North America debut for
Strikeforce at
Strikeforce Challengers 15 against
Scott Lighty, but a visa issue stemming from the recent earthquake and tsunami left him unable to leave Japan.
Lorenz Larkin took his place in the fight. In April 2011, it was reported that Ishii would retire from MMA and return to judo. Surprisingly, he said that he plans to earn American citizenship and represent the US at the
2016 Summer Olympics. However, he apparently decided against retiring from
MMA as he went on to face former
WEC Middleweight Champion Paulo Filho at
Amazon Forest Combat 1 on September 14, 2011, in his
light heavyweight debut. The fight ended in a controversial draw, with journalists as well as the crowd agreeing that Ishii had won all three rounds. Satoshi Ishii next competed at Fight For Japan: Genki Desu Ka Omisoko 2011, where he met MMA legend
Fedor Emelianenko. He lost the fight via KO in the first round.
Japanese circuit Inoki Genome Federation After taking a year off from fighting to train following the Emelianenko loss, Ishii signed an exclusive multi fight deal with IGF in Japan. Here he has secured 6 victories in less than a year against former
UFC Heavyweight Champion Tim Sylvia, UFC veterans
Sean McCorkle,
Kerry Schall,
Pedro Rizzo and against
Jeff Monson. Ishii then faced
Kazuyuki Fujita on December 31, 2013, for the
IGF Championship. He won via unanimous decision, became the first IGF Champion. Ishii defeated former
UFC fighter
Phil De Fries in a non-title bout via unanimous decision at IGF 1 on April 5, 2014, in a full pride rules match as the headline bout on the card. On August 23 at IGF 2, Despite being a heavy favorite due to his years and judo experience Ishii was defeated by
UFC,
Pride and
K-1 veteran
Mirko Cro Cop via doctor stoppage, losing the IGF Championship in the process. Ishii was expected to face
PRIDE and
Strikeforce veteran
Sergei Kharitonov at M-1 Challenge 53: Battle in the Celestial Empire on November 11, 2014. However, Ishii withdrew from the fight due to injury. Ishii faced
Mirko Cro Cop in a rematch at December 31, 2014, for the IGF Championship. Early reports had the rematch cancelled due to a rib injury sustained by Ishii. However, in December 2014, it was reported that Ishii would be able to heal up in time to fight Cro Cop, and the rematch was official once again. Ishii lost the fight via TKO at the end of the second round. After suffering back-to-back losses to Cro Cop, Ishii was scheduled to face Nick Rossborough at Inoki Genome Fight 3 on April 11, 2015. He won by a first-round submission. Ishii fought Will Penn in his final IGF appearance, at Inoki Genome Fight 4 on August 29, 2015. He won the fight by a first-round submission.
Rizin FF In his debut for the Rizin promotion, Ishii faced
Jiří Procházka on December 29, 2015, at
Rizin Fighting Federation World (8-men) Tournament. He lost the fight via knockout early in the first round. Ishii made his North American MMA debut against
Quinton Jackson on June 24, 2016, at
Bellator 157. He lost the fight via split decision. One judge scored the fight 29–28 to Ishii, while the other two judges scored it 29–28 and 30–27 for Jackson. Ishii was scheduled to face
Muhammed Lawal at
Bellator 169 on December 16, 2016. He lost the fight by unanimous decision. Following his third consecutive loss, Ishii was scheduled to face the long-time mixed martial arts veteran
Heath Herring at
Rizin 2017 in Yokohama: Sakura on April 16, 2017. He snapped his losing streak with a unanimous decision victory.
European circuit Ishii was scheduled to face Ivan Shtyrkov at RCC Boxing Promotions: Russia vs Japan on July 9, 2017. He lost the fight by a first-round technical knockout, as Shtyrkov floored the Olympian with a right straight. Ishii made his first and only Final Fight Championship appearance against Björn Schmiedeberg at Final Fight Championship 30 on October 21, 2017. He won the fight by a first-round submission. Ishii was scheduled to face Rokas Stambrauskas at German MMMA Championship 17 on October 13, 2018. He won the fight by a first-round submission.
SBC Heavyweight champion Ishii fought for the Serbian Battle Championship heavyweight title in his debut for the organization, against Tony Lopez at SBC 19 on December 1, 2018. He won the fight by a first-round submission. Ishii made his first SBC heavyweight title defense against Rodrigo Carlos at Serbian Battle Championship 20 on February 16, 2019. He won the fight by a first-round technical knockout. Ishii challenged Cally Gibrainn de Oliveira for the HEAT Heavyweight Championship at HEAT 44 on March 2, 2019. He claimed the title via second-round submission. Ishii was scheduled to face the former KSW champion Fernando Rodrigues at
KSW 47: The X-Warriors on March 23, 2019. He won the fight by split decision.
Professional Fighters League On January 1, 2019, Ishii revealed that he had signed a contract with
Professional Fighters League, and he took part in the second heavyweight regular season. Ishii faced
Jared Rosholt at
PFL 6 on August 8, 2019. He lost the fight by unanimous decision. Ishii faced
Denis Goltsov in the 2019 heavyweight quarterfinal bout at
PFL 9 on October 31, 2019. He lost the fight by majority decision.
Second run on European circuit Ishii faced Jake Heun at
Rizin 20 on December 31, 2019. He lost the fight via first-round knockout. As his first HEAT title defense, Ishii faced Cleber Souza at HEAT 46 on January 19, 2020. He retained the title via first-round submission. Ishii next faced Stuart Austin for the inaugural EMC Heavyweight Championship at EMC 5 on September 5, 2020. Ishii lost a controversial unanimous decision, to which his team decided to appeal. Ishii faced Pietro Cappelli at EMC 7 on July 3, 2021. He won the fight by a first-round submission. Ishii was scheduled to face
Daniel Spohn at HEAT 50 on May 7, 2022. However, the bout was scrapped after Spohn tested positive for coronavirus. Ishii was instead booked to face Charlie Milner at FNC 6 on June 17, 2022. He won the fight by a first-round knockout.
Return to Professional Fighter's League On June 6, 2023, Ishii announced that he had signed a new contract with
PFL in order to make his return to the promotion. He competed against
Danilo Marques at PFL 8 on August 18, 2023. He lost the fight by unanimous decision. On March 3, 2024, Ishii announced his retirement from MMA competition.{{Cite web |title=北京五輪金メダル・石井慧が引退 約14年のプロ格闘家生活で体が悲鳴「ここが潮時かな」と決断 ==Kickboxing career==