Kelvin Leerdam also scored for the defending MLS Cup champion Sounders, who moved past the Timbers in Major League Soccer's Western Conference standings.
After the teams combined for just one shot on goal in the first 60 minutes, the pace picked up. The Sounders scored all of their goals in the final 18 minutes of regulation.
Ruidiaz opened the scoring in the 72nd minute, taking a cross from Joevin Jones and flicking the ball off the crossbar from the top of the 6-yard box. The ball ricocheted off the back of the arm of Portland goalkeeper Steve Clark and trickled into the net. Nicolas Lodeiro also received an assist on the goal.
Ruidiaz scored an unassisted goal in the 83rd minute. A long clearing kick from Seattle goalie Stefan Frei deflected off a Portland defender and sent Ruidiaz on a breakaway. He chipped the ball over an onrushing Clark from about 8 yards out.
Leerdam capped the scoring in the 85th minute. Seattle's Will Bruin made a run down the left and made a cross for Ruidiaz at the top of the 6-yard box. Ruidiaz dropped a pass back to Leerdam, who beat Clark between the legs from 10 yards.
The Timbers, who were coming off a championship in the MLS is Back Tournament outside Orlando, Fla., appeared to have taken the lead after a free kick in the fifth minute on a 20-yard shot by Chris Duvall that found the upper left corner of the net. But after video review, it was determined Portland's Eryk Williamson was offside on the play.
Portland also had a goal disallowed in second-half stoppage time by an offside call.