Supporters' Shield champions LAFC had dominated throughout the season and broke records along the way, however, Bob Bradley's side came unstuck on Tuesday.
The visiting Sounders took control after Eduard Atuesta opened the scoring for LAFC in the 17th minute, with quick-fire goals from Ruidiaz and Nicolas Lodeiro putting Seattle ahead nine minutes later.
Seattle sealed their place in the November 10 final thanks to Ruidiaz's second goal of the match with 26 minutes remaining in Los Angeles – the Sounders to face either defending champions Atlanta United or Toronto in the decider.
Confidence was high at Banc of California Stadium after LAFC overcame city rivals LA Galaxy in an eight-goal thriller in the Western Conference semi-finals.
LAFC looked on track to reach their maiden final when Atuesta curled a stunning free-kick past Sounders goalkeeper Stefan Frei, however, that was as good as it got.
Ruidiaz scored a 22nd-minute equaliser following a brilliant moment of skill and Lodeiro's effort from outside the penalty area gave the Sounders the lead four minutes later.
Seattle's place in their third MLS Cup final, having won the title in 2016 and finished runners-up the following year, was secured when Ruidiaz completed his brace after the hour.