The Galaxy's Irish striker scored a huge equalizer with just 12 minutes to play, sending Seattle into Wednesday's play-in game against Colorado.
The result clinched the third seed for the Galaxy, who advance to the Western Conference semifinal against Real Salt Lake. Seattle has now gone seven games without a win, and will host the Colorado Rapids in Wednesday's play-in game, with the winner advancing to face Portland in the other conference semifinal.
Nine games into his return to MLS, Dempsey finally grabbed his first goal with the Sounders after 30 minutes. Lamar Neagle floated a cross in from the right side that Galaxy goalkeeper Jaime Penedo could only fingertip into Dempsey's path, and the USA international volleyed into an empty net with the outside of his right boot.
The Sounders went into halftime with a 1-0 lead, but that should not have been the case. Omar Gonzalez's header off a corner kick bounced over the goal-line before being cleared away, but the assistant referee failed to spot the goal, and the Galaxy were denied an obvious equalizer.
The Galaxy would get their equalizer though, and it came with just 12 minutes to play. A corner kick wasn't properly cleared, and the ball was headed back to Robbie Keane, who let the ball bounce before striking a side-volley into the turf and skipping over Michael Gspurning into the far corner.