Dallas continued its nightmare turn to the season as Portland maintained its push toward the top of the Western Conference
All three goals came in a furious seven-minute stretch in the first half. Ryan Johnson put Portland on the board in the 26th with a towering header on Diego Valeri's high, looping corner kick.
Dallas pulled one back two minutes later, with newcomer Mauro Diaz finishing off a pinpoint passing move that involved Blas Perez and Fabian Castillo.
The parity was short-lived, though. Portland won the ball in Dallas' half and countered rapidly. Valeri played a slick ball to Darlington Nagbe, who netted the winner to help the Timbers break a modest three-game winless run.
Dallas, which once topped MLS in the early days of the season and won six of its first eight games, has now failed to win a league game since May 25, a run of 11 matches. The skid has seen the club plummet out of the playoff places and into seventh, trailing fifth-place Vancouver by three points.