The 2015 Seattle Sounders FC season was the club's forty-first year of existence, and their seventh consecutive season in Major League Soccer, the top flight of United States soccer. Including previous Seattle Sounders franchises, this was the 35th season of a soccer team playing in the Seattle metro area.

The Sounders came into the 2015 season following a successful 2014, winning the US Open Cup for the fourth time as well as winning the Supporters' Shield for the first time in franchise history. By winning the Double, the Sounders became the sixth Major League Soccer team to win two major trophies in one season. This was the first time a club managed to win two trophies since the 2011 Los Angeles Galaxy. It was also the first time since the 2003 Chicago Fire for a team to do the Open Cup/League double.

Seattle also qualified for the 2014 MLS Cup Playoffs for the sixth consecutive time. Seattle came into the playoffs with the ability to become the first MLS club to win the Treble. Seattle ended up losing in the Conference Finals 2-2 on aggregate, losing by away goals.

By virtue of winning the Supporters' Shield, the Sounders qualified for the CONCACAF Champions League after a two-year drought from the competition.

Major League Soccer teams are limited to eight players without U.S. citizenship, a permanent resident (green card holder), or the holder of other special status (e.g., refugee or asylum status). These international roster slots can be traded. Sounders FC began the 2015 MLS season with eight international slots; it was announced they traded with DC United for a ninth slot during the Summer Transfer Window. Where a player has not declared an international allegiance, nation is determined by place of birth. Squad correct as of April 4, 2015.

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