Galaxy, leading 1-0 from the first leg in Seattle, doubled their aggregate advantage in the 19th minute when a Beckham corner was headed in by U.S. international striker Edson Buddle.
Eight minutes later, the former Manchester United and Real Madrid midfielder, whipped in a free-kick from the left which defender Omar Gonzalez headed past Sounders keeper Kasey Keller.
Seattle struggled to get back into the game and the 35-year-old Beckham went close to a goal himself with a trademark free-kick which curled just wide.
Beckham was sidelined for six months by an Achilles tendon injury and only returned to action for L.A in September.
"His return from injury has been phenomenal, for him to be playing like this is fantastic," said Galaxy coach Bruce Arena.
The Sounders wasted a couple of good chances after the break with Colombian Fredy Montero firing wide and Galaxy's Jamaican keeper Donovan Ricketts foiling Steve Zakuani's close range effort.
Zakuani finally got on the scoresheet in the 86th minute but by then it was far too late for a recovery from the Sounders.
Colorado and San Jose will meet in the Eastern Conference final and the winners will progress to face the Galaxy or Dallas in the MLS Cup final in Toronto on November 21.
