Harumafuji Kōhei (Japanese: 日馬富士 公平; born April 14, 1984 as Davaanyamyn Byambadorj (Mongolian: Даваанямын Бямбадорж)), previously known as Ama Kōhei (安馬 公平), is a Mongolian former professional sumo wrestler. He was the sport's 70th yokozuna from 2012 to 2017, making him the third Mongolian and fifth overall non-Japanese wrestler to attain sumo's highest rank.

Harumafuji began his professional career in 2001 and reached the top makuuchi division in 2004. He won ten special prizes for his achievements in tournaments. In November 2008 he became the seventh foreign-born wrestler in sumo history to reach the second-highest rank of ōzeki. In May 2009, he won his first tournament championship. He went on to win a total of nine top division championships, three of them with a perfect record. At 133 kg (293 lb), Harumafuji was the lightest man in the top division as of September 2015. He is noted for his technical skill and his rivalry with fellow Mongolian yokozuna Hakuhō.

Harumafuji admitted to assaulting fellow Mongolian wrestler Takanoiwa during a regional sumo tour in late October 2017. He withdrew from the November 2017 honbasho shortly after the allegations surfaced. On November 29, 2017, his stablemaster, Isegahama, submitted Harumafuji's letter of resignation to the Japan Sumo Association.

Byambadorj is a native of Ulaanbaatar, Mongolia. His father was a Mongolian wrestler, holding a rank roughly equivalent to sumo's sekiwake. Byambadorj appeared at the Naadam, a Mongolian games festival that includes wrestling, at the age of 15. He was scouted by former yokozuna Asahifuji in July 2000 and subsequently joined Ajigawa stable (now Isegahama stable).

He took the shikona or ring name Ama Kōhei and made his first appearance in January 2001. He reached the top makuuchi division in November 2004, rising slowly to komusubi in May 2006. However, a disappointing 4–11 result in this tournament sent him back to the maegashira ranks.

On the eve of the January 2007 tournament, Ama's father was killed in a traffic accident. Nevertheless, he turned in a strong 10–5 record there and earned promotion back to komusubi for March. In May 2007, he made his debut at sekiwake, becoming the ninth foreign-born rikishi to do so. He held his rank with an 8–7 record.

In September 2007, under the komusubi rank, Ama defeated new yokozuna Hakuhō on the opening day and also beat two ōzeki. He also memorably defeated newcomer Gōeidō with a spectacular technique known as okuritsuriotoshi or "rear lifting body slam." He finished with a 10–5 score and won his first shukun-shō or Outstanding Performance award. He picked up his second in November with another defeat of Hakuhō, and earned promotion back to sekiwake.

In the January 2008 tournament Ama was the only wrestler to defeat Hakuhō, during Hakuhō's successful bid for the tournament championship. This was also his third consecutive defeat of the yokozuna, and this victory helped him to win his third shukun-shō in a row. However, he failed to score more than nine wins, denting his hopes for ōzeki promotion, which normally requires three consecutive double-figure scores. After a poor start to the March 2008 tournament, he had to win his last four matches to barely preserve his rank with an 8–7 record. On the 8th day of the May 2008 tournament, he amazed audiences by defeating Wakanohō with a perfectly executed utchari or spin throw on the bales. After the match, Wakanohō acted violently and was warned. On the 10th day, Ama also threw yokozuna Hakuhō into the seats with an overarm throw, ending his winning streak. After the tournament Ama was awarded his third Technique prize.