The Sounders finished their previous season as MLS Cup runners-up, losing 2–0 to Toronto FC.

Note: Flags indicate national team as defined under FIFA eligibility rules.

[2] Note: Flags indicate national team as defined under FIFA eligibility rules.

For transfers out, dates listed are when Sounders FC officially removed the players from its roster, not when they signed with another club.

Only trades involving draft picks and executed after the start of 2018 MLS SuperDraft will be listed in the notes.