The USA national team captain scored in the 77th minute to give the Sounders a second win on the trot and a third-straight derby victory over the Timbers.
Dempsey scored his sixth goal in the past four fixtures against Portland and was thrilled to see his club move up to third position on the Western Conference table.
"I try to score every game," Dempsey said. "So anytime you can score goals, you always feel good. It's a little more special when you can score one against your rivals, especially a game-winner."
Of Seattle's recent form, which has yielded three shutouts in five outings, Dempsey added: "We're grinding out results, we're moving up the table like we should be.
"Confidence comes with getting results and I think we're growing more confident. I think our play will just continue to improve."
After the win, Sounders head coach Sigi Schmid praised former Fulham and Tottenham star Dempsey for stamping his authority on the Cascadia rivalry.
"Players get up for derby games," Schmid told reporters. "This is a derby game. Clint is trying to leave his impression."