Seattle Sounders FC is an American professional soccer club based in Seattle. The Sounders compete in Major League Soccer (MLS) as a member of the Western Conference. The club was established on November 13, 2007, and began play in 2009 as an MLS expansion team. The Sounders are a phoenix club, replacing the second-division franchise that played in the American Professional Soccer League (APSL), A-League, and USL First Division (USL-1) from 1994 to 2008, and carrying the same name as the original Sounders franchise that competed in the North American Soccer League (NASL) from 1974 to 1983.

The club's majority owner is Adrian Hanauer, and its minority owners are the estate of Paul Allen, Drew Carey, and 14 families from the Seattle area. Former USL-1 Sounders coach and assistant coach Brian Schmetzer took over as head coach in July 2016 after the departure of Sigi Schmid. The Sounders play their home league matches at Lumen Field, with a reduced capacity of 37,722 seats for most matches. Along with several organized groups, a 53-member marching band called \"Sound Wave\" supports the club at each home match. Seattle has longstanding rivalries with fellow Pacific Northwest clubs Portland Timbers and Vancouver Whitecaps FC, with whom it competes for the Cascadia Cup.

The Sounders played their inaugural match on March 19, 2009, winning 3–0 over the New York Red Bulls. Seattle has been among the league's most successful teams, winning the U.S. Open Cup four times, the Supporters' Shield in 2014, and the MLS Cup in 2016 and 2019. From its MLS inception until 2022, the team qualified for the MLS Cup playoffs—formerly the longest record in league history—and competed in the CONCACAF Champions League seven times, becoming the first MLS team to win the modern version of the competition in 2022. The Sounders were the first MLS team to participate in the FIFA Club World Cup.

The team set a new MLS record for average attendance in each of its first five seasons. The Sounders are ranked as one of the most valuable franchises in North America. Its former players have included U.S. international Clint Dempsey, long-time captain Osvaldo Alonso, and all-time assist leader Nicolás Lodeiro. The team's top goalscorer is Raúl Ruidíaz, who joined the Sounders in 2018 and surpassed Fredy Montero's record in 2024. Since 2024, the Sounders are affiliated with sister team Seattle Reign FC of the National Women's Soccer League. The Sounders also operate a players' academy and the lower-division Tacoma Defiance, which have produced homegrown players, including forward Jordan Morris and defender DeAndre Yedlin.