Martins opened the scoring in the 19th minute, and after surviving a few United chances, he netted an insurance tally in stoppage time to give the Sounders their third win in four games.
Brad Evans was the architect of the first goal with a clever through ball that put Martins in on goal, where he slotted his shot past goalkeeper Joe Willis.
Eddie Johnson had a chance to double the lead in the 63rd after playing a one- two with Martins, but Willis came off his line to snuff out the chance and keep the visitors within a goal.
Chris Pontius went down the other end and almost equalized one minute later, but goalkeeper Michael Gspurning was up to the task as he tipped a stinging shot from Pontius over the crossbar.
United continued to push for the tying goal, but a poor giveaway in the defensive third from Kyle Porter allowed Martins to get into the area and fire home the clinching goal.
07/04 00:03:23 ET