A sports league is an organization that exists to provide a regulated competition for a number of people to compete in a specific sport. At its simplest, it may be a local group of amateur athletes who form teams among themselves and compete on weekends; at its most complex, it can be an international professional league making large amounts of money and involving dozens of teams and thousands of players.

League is generally used to refer to competitions involving team sports, not individual sports. Sometimes a number of leagues are tied together in a hierarchical fashion; the best teams of one league may move to a higher league and the worst teams of a higher league may move to a lower league. This is called a league system and is most common in European and Latin American countries in sports such as football (soccer) (although it is not limited to soccer, contrary to North American popular belief ). A league may be split into smaller groups, often called conferences or divisions; teams then compete to be the best in their division and/or conference while also trying to be the best in the league.

A league championship may be contested in a number of ways. Each team may play every other team a certain number of times--usually, teams play an even number of games or matches at their own stadium and on the road, because home field advantage is a major factor in many sports. In such a set-up, the team with the best record becomes champion--based on either a strict win-loss-tie system or on a points system where a certain number of points are awarded for a win, loss, or tie, while bonus points might also be added for teams meeting various criteria. In addition to this system, many leagues rely on playoffs. Teams compete in a regular season as they would in a simple league format, but the top teams (possibly determined by conference or division) advance to the playoffs. In these leagues having the best regular season record is relatively unimportant, though top-seeded teams in some leagues, such as the NFL, can gain byes to later rounds of the playoffs, and teams finishing with the best records usually have the advantage of playing the weakest teams that have advanced to the playoffs.