The Sounders hoisted the trophy aloft last season after their penalty shoot-out win over Toronto in the 2016 showpiece.
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 after a video review for violent conduct.
The Sounders were in the box seat courtesy of goals from Gustav Svensson and Bruin in last week's opening leg in Houston.
Rodriguez virtually put the result beyond doubt in the 22nd minute – the Spaniard finishing past Dynamo goalkeeper Joe Willis having been played through on goal by Bruin.
Dempsey doubled the lead on the night approaching the hour-mark when he met Joevin Jones' low cross from inside the six-yard box.
Houston's slim hopes of salvaging the tie were extinguished when Martinez saw red after he shoved Jordy Delem in the back of the head in the 66th minute.
The Sounders showed no mercy, former Dynamo forward Bruin chipping over Willis with 17 minutes remaining.