Liam Ridgewell, Diego Valeri and Alvas Powell scored as the Timbers saw off the Galaxy 3-1 in Portland.
The Timbers made a dream start thanks to Ridgewell's fifth-minute opener — a power header into an empty net — but LA hit back immediately via Emmanuel Boateng.
Valeri restored Portland's lead just past the half-hour mark with a stunning long-range strike and Powell sealed the points when he lashed a shot across Galaxy goalkeeper Brian Rowe.
The Timbers are fifth in the Western Conference — three points adrift of first-place Sporting KC — while the Galaxy are ninth and nine points outside of the play-off positions.
Sporting KC missed the opportunity to extend its lead atop the conference after being held to a 1-1 draw by visiting Atlanta.
Benny Feilhaber's 59th-minute penalty had Sporting KC on track for back-to-back wins until Jacob Peterson netted a last-gasp equalizer as the hosts played out their fourth draw in five matches.
Atlanta United FC (@ATLUTD) August 7, 2017
Earlier, David Villa scored a hat trick to lead New York City FC to a come-from-behind 3-2 victory against the rival New York Red Bulls.