The Timbers, who won the semifinal series against the rival Sounders 5-3 on aggregate, will face Real Salt Lake in the first of the two-leg conference final on Sunday in Salt Lake. RSL defeated the two-time defending MLS Cup champion Los Angeles Galaxy 2-0 Thursday night to win the series 2-1 on aggregate.
The victory extended Portland's winning streak at Jeld-Wen Field to 16 games. The Timbers went 11-1-5 at home in the regular season with their lone loss on March 9 to Montreal. They had an MLS record 11 shutouts at home this season.
Portland pressured Seattle goalkeeper Michael Gspurning from the start. Rodney Wallace booted a shot that went just wide in the third minute, and Darlington Nagbe came close but went wide again in the 15th.
Portland broke through when Will Johnson converted a penalty kick in the 29th minute to go up 1-0.
Valeri, who had appeared to injure his shin when he was fouled hard by Seattle defender Marc Burch, came back a few minutes later with his goal from a seemingly impossible angle just before the break. Valeri was Portland's top scorer this season with 10 goals and 13 assists.
Futty Danso added a header for Portland in the 47th minute. The Sounders put up a fight, and DeAndre Yedlin's goal in the 74th minute avoided the shutout. Eddie Johnson added another three minutes later to close the gap.
The Sounders have been to the playoffs in each of the last five seasons. The Timbers, who joined MLS in 2011, are making their first trip.
The intense rivalry between the two clubs dates to 1975 when both were part of the North American Soccer league. For nearly a decade the teams have battled with the Vancouver Whitecaps for the supporter-created Cascadia Cup.
In the opening game on Saturday in Seattle, Ryan Johnson flicked in a header early in the first half and Nagbe added a goal in the second for the Timbers. Osvaldo Alonso scored late for the Sounders, who needed to win Thursday by at least two goals to become the fourth team to accomplish the comeback since the league went to the two-leg, aggregate-goal format for its conference semifinals in 2003.
Seattle was without forward Lamar Neagle due to yellow card accumulations, but Yedlin started despite an ankle injury and forward Obafemi Martins was listed as a reserve after struggling with a groin injury.
The Timbers finished the regular season in first place in the Western Conference at 14-5-15 overall. Their 23-point improvement from last season is the biggest turnaround in MLS history.
The Sounders, who were 15-12-7, defeated the Colorado Rapids in the knockout round to advance to the semifinals