Match-fixing in professional sumo is an allegation that has plagued professional sumo for decades. Due to the amount of money changing hands depending on rank, and prize money, there had been reports of yaochō (八百長) (corruption, bout-fixing) in professional sumo for years before it was finally definitively proven to exist in 2011. The hierarchical structure of the sport, in which a minority of top-ranked wrestlers have great advantages in salary, privileges and status over the lower-ranked wrestlers that make up the majority of sumo participants, may have contributed to the use of match-fixing in order to prolong careers for top-ranked wrestlers and assist in the distribution of promotions.
In The Joy of Sumo: A Fan's Notes (Charles E. Tuttle, 1991), David Benjamin determined that over a span of ten basho in the years 1989 and 1990, wrestlers who entered the final day with records of 7–7 emerged with an unlikely winning ratio of .813 (39–9). He offered further evidence of cheating in sumo, but his analysis of Day 15, "The Last-Day Blues", represents the first effort to demonstrate statistically that rikishi almost certainly trade favors late in sumo tournaments to facilitate the all-important goal of having a majority of wins in a tournament (8-7 or better, called kachi-koshi), which guarantees a wrestler will not be demoted the next tournament.
In 2002, Steven Levitt and Mark Duggan replicated and expanded upon Benjamin's research, although not crediting The Joy of Sumo. They published a paper using econometrics in order to suggest that corruption in sumo exists. Popularized in Levitt's book Freakonomics, the study found that 70% of wrestlers with 7–7 records on the final day of the tournament (i.e., seven wins and seven losses, and one fight to go) won. The percentage was found to rise the more times the two wrestlers had met, and decrease when the wrestler was due to retire. The study found that the 7–7 wrestlers won around 80% of the time when statistics suggest they had a probability of winning only 48.7% of the time against their opponents. Like Benjamin, the authors concluded that those who already have 8 wins collude with those who are 7–7 and let them win, since the 8-win wrestlers had already secured their ranking.
A possible counter-argument to the Freakonomics conclusion is that a 7–7 rikishi was highly motivated to win his last match to gain promotion, rather than demotion, while the 8–6 rikishi had already guaranteed his promotion, so was not as motivated. The authors revealed a more damning statistic, however. According to their research, the next tournament in which the two wrestlers met, there was a significant advantage to the 8–6 wrestler over the 7–7, regardless of the performance of either wrestler. The previously 7–7 wrestler would win only 40% percent of the rematches with the 8–6 wrestler. The authors suggested that winning 80% in the first match and then only 40% in the rematch (and back to the expected 50% in subsequent matches) between the same wrestlers suggested a rigging of the bouts. Additionally, the authors found that after allegations of rigging by the media, 7–7 wrestlers won only 50% of their matches against 8–6 wrestlers instead of 80%.
When the former sumo wrestler Kōnoshin Suga, also known as Onaruto stablemaster, and his supporter Seiichiro Hashimoto came forward with allegations of match rigging, drug use, sexcapades, tax evasion, and close ties to the mafia, both were found dead in the same hospital, hours apart on April 14, 1996, though there was no proof of poisoning. Weekly tabloid Shūkan Gendai alleged in a series of articles in the 1980s and 1990s that bouts were fixed.
In 2000, in both speeches and a tell-all book, former wrestler Keisuke Itai stated that up to 80% of sumo bouts were fixed. In 2007, Shūkan Gendai reported that yokozuna Asashōryū had been paying wrestlers to throw matches to him. A court later ordered Kodansha, the journal's publisher, to pay ¥44 million to the Japan Sumo Association over the allegations.
In September 2008 Wakanohō, a wrestler who had been expelled for cannabis use, claimed he was forced to accept bribes to forfeit sumo matches. He implicated ōzeki Kotoōshū, then jūryō wrestler Kasuganishiki  and later ōzeki Kaiō and Chiyotaikai. He would later retract these claims, but then changed his position in 2011 yet again, when definitive allegations of match-fixing came to light.
In February 2011, Japan's Ministry of Education, Culture, Sports, Science and Technology and National Police Agency announced that an investigation into allegations of baseball gambling by sumo wrestlers and officials had discovered cell phone text messages indicating that some matches had been fixed. Allegedly, 14 wrestlers or stablemasters were involved. Three wrestlers, Chiyohakuhō, Kasuganishiki (who had recently retired to become a coach under the elder name Takenawa), and Enatsukasa, reportedly admitted to throwing or fixing bouts. As a result of the independent investigation, the board of directors of the Japan Sumo Association (JSA) decided in an extraordinary meeting to cancel the March 2011 tournament in Osaka. The last time a Grand Tournament had been canceled was in 1946, when repairs to the old Ryōgoku Kokugikan, which had been extensively damaged in World War II, were not completed in time. Also, a regional exhibition tournament in Akita Prefecture on August 8, 2011 was cancelled.
In the end, 23 wrestlers in total were judged guilty of match-fixing and all were expelled. The JSA's investigative panel stated in May 2011 that match-fixing appears to have been widespread. The panel stated that it would be difficult to discover, however, the full extent of the problem. Uncovering the problem is made even more difficult by the presence of a separate form of collusion (koi ni yatta mukiryoku-zumo) which refers to a wrestler going lightly on an opponent without the exchange of money. In sumo culture especially, individual gain must be subordinated to the rights of the group, meaning that certain wrestlers have appeared duty-bound by their organization's style to help popular or senior wrestlers in need. Such a situation appears readily accepted by more traditional fans.
The wrestlers not listed above who admitted to match-fixing and accepted expulsion are as follows: Asōfuji, Hakuba, Hoshihikari, Kasugaō, Kiyoseumi, Kirinowaka, Kōryū, Kotokasuga, Kyokunankai, Masatsukasa, Mōkonami, Sakaizawa, Shimotori, Shirononami, Tokusegawa, Toyozakura, Wakatenro, and Yamamotoyama. Two more wrestlers, Sōkokurai and Hoshikaze never admitted any involvement and were forcibly expelled several days later. Sōkokurai was later exonerated and reinstated. See below for more information.
Soon after the match-fixing scandal, in March 2011, an 8-member committee commissioned by the JSA put forth six recommendations, later expanded to eight, for preventing future match-fixing:
Two of the wrestlers implicated by the JSA, Hoshikaze and Sōkokurai, never admitted any wrongdoing and were expelled when they refused to submit resignation papers. Both wrestlers, in separate cases took the JSA to court. Hoshikaze lost his court case in May 2012 and an appeal was denied in October of that year, with a final appeal also denied in October 2013. Sōkokurai succeeded in his case, and the JSA decided not to appeal. Sōkokurai was reinstated and re-appeared on the banzuke for the July 2013 tournament at the rank he was at when he was expelled.