Victor Rodriguez, Clint Dempsey and Will Bruin were on target at CenturyLink Field, where Seattle completed a 5-0 aggregate win.
Reigning champions Seattle Sounders secured a MLS Cup final rematch with Toronto FC after accounting for Houston Dynamo 3-0 on Thursday.
The Sounders hoisted the trophy aloft last season following their penalty shoot-out win over Toronto in the 2016 showpiece.
And Seattle will have the chance to go back-to-back against the same opposition thanks to a 5-0 aggregate win against 10-man Houston in the Western Conference play-offs.
Victor Rodriguez, United States international Clint Dempsey and Will Bruin were on target at CenturyLink Field, where Houston's Tomas Martinez was sent off following a video review for violent conduct.