A third-minute strike from Nigeria international Obafemi Martins gave the Sounders an early lead at CenturyLink Field in Washington, before Lamar Neagle tucked away a DeAndre Yedlin cross seven minutes before the interval.
The result continued a stellar period for Sigi Schmid's men, who claimed top spot in the Western Conference from the defeated Salt Lake.
Seattle also have two games in hand on each of Salt Lake and the third-placed Colorado Rapids.
The Rapids made it back-to-back victories when they came from behind to see off Dallas 2-1 at home.
David Ferreira had the visitors to Denver ahead on 51 minutes, but no less than 14 minutes later, they were trailing.
Rapids' Uruguayan front man Vicente Sanchez found the equaliser for the home side, when he made the most of a poor Dallas clearance - volleying home off his left boot for his first goal for the club.
Sanchez then had a big hand in the winner, as it was his cross that had Dallas defender Matt Hedges in trouble - with the latter getting the final touch into his own net.
The Los Angeles Galaxy failed to win for a second straight contest, as they were kept to a 2-2 draw on the road at DC United.
Goals to Robbie Keane and Michael Stephens had looked to give the Galaxy a 2-1 win, but Kyle Porter equalised for DC with six minutes to play.
Eastern Conference leaders the New York Red Bulls were fired to a 2-0 victory thanks to goals from Thierry Henry and Fabian Espindola.
Columbus Crew came from behind to beat Montreal Impact 2-1 in Canada, with Chad Marshall and Dominic Oduro the goalscorers for the winners.
The Houston Dynamo had a much-needed 1-0 win away at the Philadelphia Union, to consolidate their spot in the top five in the West.
The Chicago Fire picked up four points in five days, following up a 1-1 draw in Toronto on Wednesday with a 3-2 come-from-behind win at home to New England.
Alex scored in the 86th minute to hand the Fire their 11th win of the season, which leaves them goal differential outside the East's top five.
Chivas USA and the Portland Timbers played out a 1-1 stalemate, while there were no goals in the San Jose Earthquakes' hosting of the Vancouver Whitecaps.
