Jordan Morris scored in the 56th minute of the Western Conference final second leg Sunday, giving the Sounders a 1-0 win on the day and a 3-1 aggregate triumph over the Colorado Rapids.
Sounders striker Nelson Valdez slipped in Morris, who beat a charging Zac MacMath to the ball and lifted an outside-of-the-foot shot into the back of the net.
The Sounders got goals from Morris and Nicolas Lodeiro to claim a 2-1 win in the first leg Tuesday before finishing the job by handing the Rapids their first home defeat of 2016.
The MLS Cup final will mark the culmination of a remarkable run by the Sounders, who sat ninth place in the Western Conference when longtime coach Sigi Schmid was replaced by Brian Schmetzer on July 26.
Despite losing Clint Dempsey to an irregular heartbeat down the stretch, Seattle went 8-2-4 following the dismissal of Schmid and midseason signing of Lodeiro. The fourth-seeded Sounders then earned a 1-0 win over Sporting Kansas City in a knockout round match before notching a 4-2 aggregate victory against FC Dallas in their conference semifinal series.