Dempsey and Obafemi Martins were both denied by Portland goalkeeper Donovan Ricketts earlier in the sequence with sprawling kick saves. The ball eventually landed at the feet of Gonzalo Pineda and his diagonal pass found Dempsey open in front of the net for his first goal with Seattle since April 26 against Colorado. Dempsey was kept onside on the play by Portland's Will Johnson.
Seattle defeated Portland 3-1 in extra time in the quarterfinals of the U.S. Open Cup earlier this week and moved 10 points clear of second place Real Salt Lake in the MLS Western Conference with the victory.
Marco Pappa added his second goal of the season in the 86th minute on a counter attack moments after Portland's Gaston Fernandez hit the outside of the goalpost with his attempt. It was Pappa's second goal this week against the Timbers after scoring the final tally in the U.S. Open Cup victory.
Dempsey's return after two months away for World Cup duty came before a crowd of 64,207, just shy of a sellout at CenturyLink Field. It was the only visit to Seattle this season by the Timbers.
While there lacked the fireworks of the 4-4 draw the sides played in Portland in early April, there was plenty of chances in the final 45 minutes and most of them coming at Seattle's offensive end.
Ricketts was tested repeatedly, including the second minute of the second half on a shot by Pineda. He smothered multiple shot attempts by Federal Way's Lamar Neagle and denied Dempsey off a quick throw-in in the 55th minute when the United States captain tried to sneak a right-footed shot past Ricketts.
But Ricketts could only watch as Dempsey scored his ninth goal of the season, including four coming against the Timbers. Dempsey scored a hat trick during the first matchup in April.
Seattle goalkeeper Stefan Frei was rarely challenge, but came up with a key pair of saves three minutes into the second half. He first blocked former Sounder Steve Zakuani's shot from the top of the penalty area, then deflected the rebound attempt by Fanendo Adi.
Portland is now winless in its last four matches, while Seattle is undefeated in six of its last seven league games.