Debutant Leo Chu, on as a substitute, provided the through ball service on Ruidiaz's winner, which sets up Seattle to face the winner of Wednesday's semifinal between Pumas UNAM and Club Leon in September 22's final in Las Vegas.
Ruidiaz saw his first attempt on the 93rd-minute break saved by Gibran Lajud, but was first to the rebound to guide it across the goal line.
It was Ruidiaz's second goal in the Leagues Cup and 16th in all competitions for the Peruvian, who is also leading the 2021 MLS Golden Boot presented by Audi race.
The goal provided a deserved result on a night where the Sounders generated more clearcut chances, even if both teams appeared to run out of steam late.
Ruidiaz came within inches of completing a give-and-go goal with Cristian Roldan a few minutes before halftime. Shortly after the break, Fredy Montero fired a volley over the bar after running onto Kelyn Rowe's excellent ball to the back post. And Yeimar Gomez Andrade fired a volley off a corner kick over the bar in the 62nd minute.
Santos came closest to scoring when Diego Valdes struck the right post with a 65th-minute free kick. The Liga MX side controlled more of the possession as the second half wore on, but failed to translate that into many clear chances.
- 90'+3' - SEA - Raul Ruidiaz | WATCH
- THE BIG PICTURE: The victory puts the Sounders in position to fill what's arguably the only hole in their résumé in the battle for most successful MLS clubs of all-time. Should they win next week, they would join D.C. United, the LA Galaxy and Atlanta United as MLS teams to win continental honors. During their MLS era, the Sounders have won two MLS Cups, four U.S. Open Cup crowns and one Supporters' Shield.
- MOMENT OF THE MATCH: It's hard to explain exactly why Ruidiaz found so much space deep into second-half stoppage time, but he did. And as he has proven all season, the 31-year-old wasn't likely to waste such an opportunity.
- MAN OF THE MATCH: Raul Ruidiaz was the most menacing creative force on either team over the course of the evening. His match-winner just made this call a lot more decisive.
- SEA: Saturday, Sept. 18 at Real Salt Lake | 9:30 pm ET (MLS LIVE on ESPN+)