A rikishi (力士), sumōtori (相撲取り) or, more colloquially, osumōsan (お相撲さん), is a sumo wrestler. Although used to define all wrestlers participating in sumo wrestling matches, the term is more commonly used to refer to professional wrestlers, employed by the Japan Sumo Association, who participate in professional sumo tournaments (called honbasho) in Japan, the only country where sumo is practiced professionally.

Professional rikishi follow traditions dating back to the Edo period, and therefore follow a number of codes and customs in their daily lives that distinguish them from other martial artists. Their life as professionals revolves around the observance of traditional rules that apply both to their life in the community and to the way they dress, the latter rules evolving according to the rank a wrestler has reached during his career.

Coming from many different nationalities, rikishi are the only employees of the Japan Sumo Association who can run the organization once they have chosen to retire. However, only a tiny fraction of wrestlers are given this opportunity, leaving the vast majority of the sport's retirees in a precarious situation.

The number of active rikishi peaked at 943 in May 1994, at the height of the "WakaTaka boom", but had declined to 599 by January 2024. The decline in the number of recruits applying to become professionals is a major topic in sumo, as it regularly breaks records for the lowest number of recruits.

In popular use, the term rikishi refers to professional sumo wrestlers only and is an alternative term to sumotori (相撲取り, sumōtori; lit. 'sumo practitioner') or the more colloquial osumosan (御相撲さん or お相撲さん, osumōsan; lit. 'honorable Mr. Sumo'). It has been noted by authors such as Dorothea Buckingham and Mark Schilling that these terms should be preferred to 'sumo wrestler', because since sumo has little in common with Greco-Roman wrestling but more with judo or aikido, it was pointed out that it was a mistake to use the term 'wrestler' to define the competitors in sumo matches.

The two kanji characters that make up the word rikishi are that of 'strength' or 'power' (力) and 'warrior' or 'samurai' (士); consequently, and more idiomatically, the term can literally be defined as 'strongman' or 'powerful warrior'.

Sumotori is often defined as the more appropriate collective name for the wrestlers as a group or as individuals. The term itself comes from an abbreviation of the word sumo no toride (相撲の取手), used in the early Edo period to define sumo wrestlers. There is no implication of hierarchy between rikishi and sumōtori, the two terms being interchangeable. However, some wrestlers prefer to be referred to as osumōsan.

A more prestigious term referring to wrestlers who have risen to the two highest divisions (jūryō and makuuchi) also exists. The word sekitori refers to senior rikishi who have significantly more status, privilege and salary than their lower-division counterparts and excludes the lower-rankers. Wrestlers who qualify as sekitori are also given the suffix -zeki (関【ぜき】; lit. 'barrier') at the end of their name. That term, found also in the sumo terms sekitori (関取), ōzeki (大関) and sekiwake (関脇), comes from sekisho (関所), a road barrier which was used to control the movement of people from place to place within Japan. In feudal Japan, many wrestlers were recruited from the big, strong guards who manned the sekisho. Later "-zeki" came to mean an unbeaten performance.