Martins opened the scoring on a header off a Mauro Rosales free kick and the Sounders (9-7-4) rolled on from there to chalk up a club-record 10th straight home game without a loss.
Eddie Johnson tallied in the 22nd minute and Brad Evans added a penalty kick goal late in stoppage time.
Dallas (8-6-8) remained mired in a nine-game winless streak.
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