Seattle took the lead in the 16th minute when Steve Zakuani robbed the ball in midfield and countered upfield. His pinpoint cross was turned in by Eddie Johnson from close range.
Portland keeper Donovan Ricketts was called on to make a good save minutes later to deny Johnson scoring again.
After the break, the Timbers should have been level only for the post to deny a Valeri free kick which had Gspuring beaten all ends up.
In a half of few chances the visitors tried to press to get level but were kept at bay until the 90th minute when Wallace headed Andrew Baptiste's cross into the top corner.