The Sounders (1-3-2) got the game's only tally when Colorado's Clinton Irwin made a diving save on a Shalrie Joseph volley, but Martins was there to fire home the rebound.
It was just the third goal for the season for Seattle.
The Rapids (2-4-2) had their best chance in the 58th went Sounder goalkeeper Michael Gspurning was forced to make a diving save on header by Danny Mwanga.
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