Robbie Keane got the only goal for Los Angeles in its 2-1 loss to Seattle in the second leg of the two-game Western Conference Finals at CenturyLink Field, but because they entered with a 3-0 lead, the Galaxy advanced.
Los Angeles defeated the Dynamo 1-0 in the 2011 MLS Cup, and will host this year's Dec. 1 title match by dint of their edge over Houston in regular-season points.
The Dynamo bested D.C. United in the Eastern Conference final by an aggregate score of 4-2 earlier Sunday.
Eddie Johnson and Zach Scott scored for the Sounders in their second-leg victory.
Notable deaths of 2014 [PHOTOS]
Julius Randle breaks leg during NBA debut