Dempsey entered the match as substitute in the 34th minute to replace the injured Obafemi Martins, but the American international had little impact on the outcome.
Already holding a one-goal lead thanks to an early strike from Mauro Rosales, the Sounders received what proved to be the game-winning tally from Doneil Henry's own goal in the 40th minute. Jonathan Osorio scored early in the second half to give the Reds some hope, but Seattle managed to close out the match for a third successive win.
It took the visitors just 16 minutes to break the deadlock as Eddie Johnson headed a cross back across goal, allowing Rosales to take advantage of some statuesque Toronto defenders and fire home the opener.
The Sounders grabbed another goal just prior to the halftime interval when Lamar Neagle created space on the left flank before Henry turned a low cross into his own net.
It proved to be a crucial goal as Osorio cut the deficit to one goal by firing home a low cross that hit the post and narrowly crossed the goal line.
But Toronto failed to equalize down the stretch, bringing an end to the club's two-match winning run.
08/10 21:24:13 ET