Lamar Neagle scored the only goal of the game at CenturyLink Field in Washington as the Sounders claimed their third straight win.
Real Salt Lake top the conference by five points but have played three games more than Seattle, who are level on points with the Los Angeles Galaxy with a game in hand on the Californians.
Even without star signing Clint Dempsey, who is away on international duty with the United States, and injured former Inter striker Obafemi Martins, Sigi Schmid's side secured another win.
A Marc Burch set-piece found Neagle in the 24th minute and the striker took a touch before volleying past Dan Kennedy from close range.
In Wednesday's other game, Houston Dynamo missed a chance to move into the top five after a 2-0 loss against Columbus Crew.
Second-half goals from Ryan Finley and Justin Meram secured the victory for the hosts, who are eighth in the Eastern Conference.
Dynamo six sixth, equal on points with fifth-placed New England Revolution, after their third consecutive game without a win.
Finley opened the scoring on 47 minutes after a neat team move led to Meram, squaring a pass for the former to tap into an open net.
Columbus had a chance to double their advantage just three minutes later when they were awarded a penalty after Robert Boswell pulled down Dominic Oduro.
But Tally Hall kept Houston in the game, saving Finley's poor penalty which went straight down the middle.
The Crew scored their sealer in the 74th minute as Meram beat a defender before finishing into the bottom corner from inside the area.
