Seattle (12-15-3, 39 points) has won consecutive games for the first time since June.
Austin (15-9-6, 51 points) was shut out for a second straight game and has lost three in a row for the first time this season.
Ruidiaz nearly got the home side on the board in the fifth minute with a long shot from the top of the box, but he was stopped by goalkeeper Brad Stuver.
Seven minutes later, he connected from close range to give the Sounders a 1-0 lead.
Jordan Morris, along the right wing, sent the ball through an Austin defender to Nicolas Lodeiro in the box. Lodeiro dished off to Ruidiaz to his left at the top of the goal area, where the Peruvian one-timed it past Stuver.
Seattle kept the pressure on, eyeing another tally midway through the half.
In the 25th minute, Lodeiro maintained the ball and cut toward the net along the goal line looking for a pass but was blocked. A minute later, Danny Leyva fired a shot from distance but was denied by Stuver, who jumped to snare the attempt.
Ruidiaz made it 2-0 in the 34th minute. Morris won a foot race to the ball just outside the box and took it inside before feeding Ruidiaz at the top of the goal area for a quick strike.
Austin had its best chance of the first half in the 37th minute off a set piece. After the initial kick was blocked by the wall and the Sounders blocked the rebound attempt, defender Julio Cascante nearly got the visitors on the board from right in front, but his shot trickled just past the post.
Seattle benefited from an own goal to push it to 3-0 in the 68th minute when Lodeiro's free kick deflected off the back of Austin defender Ruben Gabrielsen and into the net.