Goalkeeper Stefan Frei made two saves to post his second shutout in three games for the Sounders, who moved into first place in the Western Conference.
The Galaxy suffered their first defeat after opening the season with two straight wins for the first time since 2010.
Los Angeles' Javier "Chicharito" Hernandez was held off the scoresheet for the first time this season. Hernandez had scored five times in the first two games, equaling the MLS record set by Houston's Brian Ching in 2006. Hernandez failed to match or surpass Chicago's Ante Razov, who set the three-game record of six goals in 1999.
Ruidiaz and Smith scored three minutes apart midway through the first half.
Ruidiaz opened the scoring in the 20th minute, taking a cross from Smith on the left wing and volleying the ball from 10 yards out into the right side of the net, just out of the reach of diving Galaxy goalie Jonathan Bond.
Smith struck in the 23rd minute. He took a cross from Cristian Roldan on the right wing and put a shot on net that was stopped by Bond. But as two Galaxy defenders watched, Smith ran past them and knocked in his own rebound from five yards out.
Ruidiaz capped the scoring in second-half stoppage time, taking a cross from Jimmy Medranda and putting a left-footed shot past Bond from eight yards out.
Frei's best save came in the 27th minute, as Los Angeles' Efrain Alvarez sent a pass over the Sounders' defensive line to teammate Samuel Grandsir at the top of the 18-yard box. Grandsir settled the ball but his shot from 15 yards out was deflected wide of the net by a charging Frei.
Sounders captain Nicolas Lodeiro made his season debut as a substitute in the 66th minute after missing the first two matches because of a strained left quadriceps.
The Sounders wore their alternate Jimi Hendrix kit for the first time.