With 94 minutes played and time running out at Providence Park on Sunday, United States international and substitute Dempsey popped up to steal a point for the visiting Sounders.
The Timbers, on a three-match losing streak, were on track after Fanendo Adi and Dairon Asprilla struck in first-half stoppage time.
Joevin Jones followed up his own rebound to give the Sounders a 27th-minute lead away from home.
Adi restored parity from the penalty spot for his 50th MLS goal after Darlington Nagbe was fouled inside the area, with Brad Evans shown a red card.
It got worse for the Sounders just moments later when Asprilla rose highest to head home David Guzman's corner on the stroke of half-time.
But as the Sounders neared victory, Dempsey came off the bench and guided a header into the bottom corner of the net.
The slumping Timbers are third in the Western Conference, four points ahead of eighth-placed Seattle.