Ruidiaz's right-footed shot from 41 yards out soared over leaping Austin FC goalkeeper Brad Stuver and into the center of the goal for the match's lone tally, granting Seattle three points on the road.
The Sounders (9-1-5, a MLS-leading 32 points) bounced back after a 1-0 loss at Minnesota United on Sunday that snapped their MLS-record 13-match unbeaten run to start a season. The Sounders have not lost two straight games since May-June 2019.
Seattle was forced into an extremely young and inexperienced lineup when 10 of its regular players were unavailable due to international call-ups, injuries and suspensions.
The Sounders became the first MLS club to start five teenagers in a match, and 15-year-old Obed Vargas became the third-youngest player to appear in an MLS game. The average age of the Sounders' starters was 22 years, 7 months, making the group the second-youngest starting lineup in MLS history.
Austin FC (3-7-4, 13 points) lost its second straight game. The team has just one win in its past 11 matches (with six losses and four draws) after winning two of its first three games in its inaugural season.
Julio Cascante seemingly produced the equalizer in the 77th minute for Austin on a diving header off a set-piece pass by Cecilio Dominguez, but the goal was reversed after a video review showed that Cascante was offside.
Austin played its first match in 15 days, with Thursday's game the start of a busy stretch for the expansion side as it plays five matches over a 17-day span. Austin FC also was short-handed, playing without seven players from its regular roster.
Austin pressed the attack in the opening minutes of the second half and had an excellent chance to score in the 52nd minute. Alexander Ring's header off a centering pass from Diego Fagundez was gobbled up by Seattle goalkeeper Spencer Richey, keeping the home side off the scoreboard.
Richey, starting his first match of the season, ended up with three saves to record the fifth shutout of his six-year MLS career.