Stefan Frei made three saves while recording his first shutout of the season for the Sounders (4-5-1, 13 points), who are finding their stride after winning the prestigious CONCACAF Champions League title earlier this month.
Steve Clark made three saves for Houston (4-5-3, 15 points), which is 1-4-1 over its last six matches.
The Dynamo played a man short for 28 minutes in the second half after Adalberto Carrasquilla was given a red card and ejection for drawing his second yellow card of the match in the 54th minute.
But Seattle's Alex Roldan was carded for tripping in the 82nd minute. That was his second yellow of the match to earn the red and an ejection, leaving the two sides playing 10-on-10 for the final eight minutes of regulation and three stoppage-time minutes.
The Dynamo held a 14-9 edge in shots but the Sounders put four on target to Houston's three.
The Sounders took the lead in the 28th minute following a beautiful passing sequence. The finale of the move began with Jordan Morris to the left of the net, passing the ball through the goal mouth to Roldan on the right. Ruidiaz was nearby and received the pass from Roldan and let go a close-range shot that caromed off the leg of Clark and hit the right goal post before finding the net for his second of the season.
Houston had a solid chance to knot the score in the 78th minute but Tyler Pasher's hard left-footed strike sailed over the crossbar.
Seattle nearly took a 2-0 lead in the 90th minute, but Will Bruin's header was wide to the right.
In the first half, Houston's Darwin Quintero had two solid chances. His left-footed blast in the 14th minute was too high and his close-range shot in the 23rd minute was saved by Frei.