Houston Dynamo and LA Galaxy will contest the MLS Cup final for a second successive season after both held first-leg leads to clinch their Conference championships on Sunday.
• Highlights: Sounders v Galaxy
• Highlights: DC United 1-1 Houston
Galaxy, who beat Seattle 3-0 last week, were largely outplayed as they lost 2-1 but progressed, while Houston - 3-1 up after their home game against DC United - secured a 1-1 draw to clinch a comfortable aggregate win.
The teams will face each other at Galaxy's Home Depot Center stadium on December 1, with Houston hoping to avenge last year's 1-0 defeat.
Galaxy, featuring David Beckham and Robbie Keane in their ranks, will feel relieved to have emerged from the second leg unscathed after falling 2-0 behind as Seattle appeared poised to complete a remarkable revival.
Seattle forward Eddie Johnson, who had already had a goal debatably chalked off for offside, struck early on - and when Zach Scott doubled the advantage before an hour had been played, Galaxy were on the ropes.
But a Keane cross was adjudged to have hit the hand of defender Adam Johansson, and the Irish forward converted the 68th-minute penalty to deny the Sounders and secure the Western Conference title.
A relieved Keane, who said: "It's about getting to the final, and we did that" admitted Seattle had been the better side.
Houston had an easier time, all but securing their place in the final before half time when Brad Davis set up Boniek Garcia. Branko Boskovic grabbed an equaliser, but it proved to be too little, too late.