Sounders captain Clint Dempsey and striker Obafemi Martins combined for three goals at Providence Park as the Western Conference leaders sent an ominous warning to the rest of the competition.
The win - Seattle's first in two games - helped the Sounders move to 45 points from 24 rounds, three clear of Dallas, who have played an extra match.
Portland's defence struggled to cope during the opening stages, and they were eventually breached in the 18th minute after Dempsey hit the post moments earlier.
After Diego Valeri's attempt on goal deflected over the crossbar for a corner on the half-hour mark, Seattle went up the other end and doubled their advantage following a swift counter-attacking move in the 34th minute.
Martins picked up the ball in his own half, pressed forward and waited for Dempsey's run before playing in the United States international, who tucked the ball past Timbers goalkeeper Donovan Ricketts for his ninth goal of the season.
Portland were given a glimmer of hope, when substitute Fanendo Adi got a boot to Rodney Wallace's cross with 27 minutes remaining.
Martins, however, quickly dashed Portland's hopes of a remarkable comeback as he netted his second of the game three minutes later.
