The reigning champions led 3-0 after the first leg at The Home Depot Center last weekend and it appeared that the Sounders had worked their way back into calculations on Sunday when they went two-nil up in the return bout.
However, the Galaxy won a penalty which was converted by Robbie Keane with just over 20 minutes remaining to see the Sounders take the fixture 2-1 but allowing LA to move into the Final for the second season in a row.
Galaxy skipper Landon Donovan was full of praise for his teammates after a tough aggregate victory whilst star midfielder David Beckham felt it added another string to the clubs bow to get to the decider the hard way.
"We've got a bunch of champions in this locker room and even though going down a goal, going down a second goal, we were still resilient and got the goal that kind of broke their back," said Donovan.
Beckham added: "Every success that the club has is obviously rewarding, whether we've done it the easy way or the hard way.
"Tonight was the hard way, but this season's been the hard way."