He needed to make just one save on a shot by Osvaldo Alonso in the 90th minute.
Rimando also set up the game's lone goal with a long pass to Robbie Findley just steps outside the opposite box in the 62nd minute.
RSL rose to 7-8-5, winning for the sixth time in 10 home games, and closed the gap on first-place Houston to nine points despite being in sixth place in the Western Conference.
The Sounders dropped to 6-9-5 after their second straight setback.
They fell to fourth from second in the West.