Philly moves top of the East in a win over Seattle that hands the LA Galaxy the Supporters' Shield.
Freddy Adu and Brian Carroll scored the goals to move Philly top of the Eastern Conference.
"We'll take it on the chin and we'll learn from it," Seattle coach Sigi Schmid said.
Seattle had multiple opportunities to score. Brad Evans clinked a deft chip off the crossbeam early in the second half for the Sounders' closest chance. The Union duly made them pay.
Gabriel Farfan hustled James Riley at the endline, poking the ball back into the box when the Sounders fullback thought he'd done enough to run it out of play. There, Adu slammed a shot into the roof of the net for his second Philadelphia goal.
"I was falling down and I had to concentrate on getting a good foot on it," Adu said. "The only place I could hit it was going up, trying to hit the ball to the roof of the net. I knew Kasey [Keller] was there but when you're falling down you want to hit the ball as hard as you can."
With Seattle pushing for the equalizer, Philly hit on the counter 10 minutes later. Former Sounder Sebastien Le Toux led the charge, cleverly laying off for Carroll at the end of his run. The defensive midfielder slotted a tight shot past Keller to ice the match.
"It was just one of those bad days," Keller said. "Everybody can look at whatever different excuse you want to come up with but at the same time we just weren't good enough today."
Jeff Parke suffered a concussion and had to be replaced at halftime.
The result hands the LA Galaxy the 2011 MLS Supporters' Shield for the best record in the regular season.