The Galaxy trailed to an Omar Gonzalez own goal but fought back through two goals from Landon Donovan either side of half-time - the first a penalty and the second a tap-in after a smart exchange with Chad Barrett.
Colombian Juan Pablo Angel made it 3-1 before the 36-year-old Beckham curled in a 20 yard free-kick from the left to complete a comfortable victory.
A crowd of over 36,000 turned out at Qwest Field in Seattle to watch the Sounders take on local rivals Portland in the first top flight meeting of the two clubs since the days of the old NASL.
A left foot drive from Alvaro Fernandez put the Sounders in front in the 51st minute but Futty Danso headed in a Jack Jewsbury free-kick in the 64th minute to earn John Spencer's Timbers a point.
Real Salt Lake's hangover from their CONCACAF Champions League Final defeat continued with a disappointing goalless draw at home to the Houston Dynamo.
A Shalrie Joseph penalty gave the New England Revolution a 1-0 win over the Vancouver Whitecaps while Dallas enjoyed a 2-0 victory over the Philadelphia Union thanks to a close-range finish from Brek Shea and a Fabian Castillo header.
