Martinez, a former Honduran national team member playing in his first season for San Jose, was denied by Sounders goalkeeper Michael Gspurning on a bicycle-kick try in the 14th minute.
But after Gspurning left after the first half with a wrist injury, Martinez bested backup goaltender Andrew Weber in the 48th minute with a bending shot, enabling San Jose (6-9-6) to break a two-match losing streak.
Seattle (7-7-3) fell to .500.
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