Major League Soccer (MLS) is a men's professional soccer league sanctioned by the United States Soccer Federation which represents the sport's highest level in the United States and Canada. The league comprises 26 teams—23 in the U.S. and three in Canada—and will expand to 30 teams by the 2023 season. MLS constitutes one of the major professional sports leagues in both countries.

The MLS regular season starts in late February or early March and runs through mid-October, with each team playing 34 games; the team with the best record is awarded the Supporters' Shield. Fourteen teams compete in the postseason MLS Cup Playoffs in late October and November, culminating in the league's championship game, the MLS Cup.

Major League Soccer was founded in 1993 as part of the United States' successful bid to host the 1994 FIFA World Cup. The first season took place in 1996 with ten teams. MLS experienced financial and operational struggles in its first few years as the league lost millions of dollars and folded two teams in 2002. Since then, developments such as the proliferation of soccer-specific stadiums around the league, implementation of the Designated Player Rule allowing teams to sign star players such as David Beckham, and national TV contracts have made MLS profitable. With an average attendance of over 20,000 per game, MLS has the third-highest average attendance of any sports league in the U.S. after the National Football League (NFL) and Major League Baseball (MLB), and is the seventh-highest attended professional soccer league worldwide.

Instead of operating as an association of independently owned clubs, MLS is a single entity in which each team is owned by the league and individually operated by the league's investors. The league has a fixed membership like most sports leagues in the United States and Canada, which makes it one of the world's few soccer leagues that does not use promotion and relegation. MLS headquarters is located in New York City.