New addition Mike Chabala scored in the 26rd minute and Jorge Perlazza and Eric Brunner added goals for Portland, who played aggressively and was backed by its usual boisterous crowd, peppering Galaxy goalkeeper Josh Saunders with 15 shots.
In contrast, the Galaxy barely caused Portland keeper Troy Perkins to break a sweat, with the majority of their seven shots on goal coming in the final 10 minutes.
The league-leading Galaxy were attempting to tie a franchise record with their 15th straight game without a loss but even the return of David Beckham from suspension could not prevent defeat.
Sporting Kansas City 2, Real Salt Lake 0 In Kansas City, Roger Espinoza scored a rare goal before getting red-carded, and Sporting Kansas City hung on with ten men to beat Real Salt Lake and extend its club-record unbeaten streak to 14 games.
Teal Bunbury added his second goal in as many games for Kansas City, which moved level with New York for third place in the Eastern Conference.
Espinoza, whose hard tackle sparked a bench-clearing brawl in a preseason game between the two clubs, was sent off for a hard tackle on Kyle Beckerman in the 57th minute.
Real lost back-to-back games for the first time this season.
Chicago Fire 1, Philadelphia Union 1 In Bridgeview, Illinois, midfielder Pavel Pardo scored in his Philadelphia debut to give the Union a share of the points at Chicago.
Former Mexico international Pardo beat goalkeeper Faryd Mondragon with a 17-yard shot nine minutes into the second half.
The score erased Philadelphia's lead, which had come via Veljko Paunovic's 30-yard shot in the 34th minute.
The point moved the Union within two points of Eastern Conference leader Columbus.