Urruti broke the scoreless tie when he got his foot on a corner kick from Will Johnson and steered it past Galaxy goalkeeper Jaime Penedo in a downpour at JELD-WEN Field.
The Timbers (12-5-13) solidified their hold on third place in the Western Conference, moving four points clear of the Galaxy.
Los Angeles (13-11-6), meanwhile, saw its winless streak extend to four league games.
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