MLS Cup 2017 was the 22nd edition of the MLS Cup, the championship of Major League Soccer (MLS), contested between Toronto FC and Seattle Sounders FC on December 9, 2017. The soccer match was hosted at BMO Field in Toronto, Canada, for the second consecutive year and third time overall. Both teams made their second appearance in the MLS Cup, in a rematch of the 2016 edition.

Seattle Sounders FC is an American professional men's soccer club based in Seattle, Washington, United States. 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.