Cristian Roldan also scored for the Sounders, who remained a point ahead of New England in the Supporters' Shield standings for the league's best record. The Revolution defeated the New York Red Bulls 3-2 earlier in the night.
Albert Rusnak scored on a penalty kick in the 79th minute for Real Salt Lake, which had its unbeaten streak snapped at four matches.
Ruidiaz's league-leading eighth goal of the season came after RSL's Anderson Julio was called for a hand ball in the penalty area. Ruidiaz chipped the ball straight down the middle to beat goalkeeper David Ochoa.
That came nine minutes after RSL tied the score at 1-1 on Rusnak's penalty.
It came after Seattle goalkeeper Stefan Cleveland, making his fifth consecutive start for the injured Stefan Frei, was called for tripping Rubio Rubin as he took a cross just outside the top left of the 6-yard box. Rusnak blasted his penalty into the left side of the net, with Cleveland only able to get a fingertip on the shot.
The Sounders outshot RSL 9-2 in the first half but put only two of their attempts on goal, both saved by Ochoa.
Roldan opened the scoring in the 58th minute.
Sounders midfielder Jimmy Medranda knocked the ball away from RSL's Nick Besler with a sliding tackle from behind near midfield. Seattle's Joao Paulo gained possession and immediately put a pass straight up the field for Roldan. He passed to Ruidiaz, who dribbled the ball near the top right of the 18-yard before sending a cross back for Roldan near the penalty spot.
Roldan's one-touch pass created an opening for Will Bruin, whose 10-yard shot was stopped by a diving Ochoa. But the rebound came right back to Roldan, who hammered an 8-yard shot into the net.