Komusubi (小結, komusubi) literally means "the little knot", the knot referring to the match-up between two wrestlers. It is the fourth highest rank in sumo wrestling and is the lowest of the so-called titleholder ranks, or san'yaku.

It is also the lowest rank where achieving a kachikoshi (or majority of wins) is no longer sufficient to guarantee promotion to a higher rank. Promotion to the next highest rank, sekiwake, depends on either a space being available, which is quite common, or having a record in the previous tournament that is very convincing, typically 10–5 or better.

For many purposes this and the sekiwake rank are treated together as the junior san'yaku ranks, as opposed to ōzeki and yokozuna, where extremely stringent promotion criteria exist. Records of number of tournaments ranked in junior san'yaku are often referred to in sumo publications because these two ranks are so difficult to retain.

For wrestlers reaching this rank the benefits are a salary increase and also appearing to flank the chairman of the Sumo Association during the speeches he makes on opening and closing days of the official tournaments, held six times a year. He may also be called on to represent the wrestlers on behalf of the Sumo Association at other events, especially if the number of ōzeki and yokozuna are low. If this is the highest rank a wrestler reaches, even if it is only for one tournament, he will always be referred to as "former komusubi (ring name)" after his retirement, which is an indicator of a fairly successful sumo career.

At any time there must be a minimum of two wrestlers ranked as komusubi. If circumstances require this can rise, typically to three or four. The minimum of two requirement means that a certain amount of luck can lead to wrestlers achieving this rank on occasion, if the performance of other wrestlers leaves no obvious candidates to fill the rank.

Komusubi is widely regarded as a difficult rank to maintain, as wrestlers at this rank are likely to face all the ōzeki and yokozuna in the first week of a tournament, with a yokozuna normally scheduled for the opening day. Komusubi face mainly maegashira in the second week, but often wrestlers new to the rank are so demoralised by this point that they lose these matches too. Few men making their komusubi debut return a kachi-koshi or winning score.

Before World War II there were several instances of komusubi immediately advancing to ōzeki after nearly winning a tournament, but there have been no instances of this since then.