Portland's Julio Cascante deflected the ball into his own net in the 76th minute as the host Timbers suffered a 1-0 loss to the longtime rival Seattle Sounders on Sunday night.
Goalkeeper Stefan Frei made three saves for his sixth shutout of the season for the Sounders, who won their franchise-record seventh consecutive game and stretched their unbeaten streak to 10 matches.
The Sounders also moved past the Timbers in the race for the Western Conference's sixth and final playoff berth.
The Timbers suffered their fourth straight defeat after a 15-match unbeaten run.
The own goal came after Sounders defender Kim Kee-hee made a run down the right wing.