The Galaxy lost 2-1 to Seattle in the second leg of the Western conference final, but advanced 4-2 on aggregate after winning the first leg 3-0.
The reigning champions will again face Houston Dynamo in the decider, after the Dynamo also won through 4-2 on aggregate after a 1-1 draw with DC United in the second leg of the Eastern Conference final.
Despite denials from the man himself, Beckham has been linked with a possible guest stint in the A-League, though any negotiations will likely have to wait until after the MLS season.
The Galaxy were in a dominant position after the first leg but the Sounders made it clear early they didn't intend to go quietly.
Twelve minutes into the contest, Eddie Johnson held off Galaxy defender Omar Gonzalez and slipped a shot past Los Angeles keeper Josh Saunders and when Zach Scott made it 2-0 in the 57th minute, the Galaxy's aggregate lead was down to just one goal.
Ten minutes later however referee Mark Geiger awarded Los Angeles a penalty when Ireland international Robbie Keane's attempted cross was blocked by the outstretched arm of Adam Johansson.
Keane calmly converted the penalty in the 68th minute - giving the Galaxy breathing room with what proved to be the final goal of the night.
Because they finished the regular season with more points than Houston, the Galaxy will host the Dynamo at the Home Depot Center in the MLS Cup final on December 1.