Wallace sped in on goal after receiving a pass from Diego Chara, shook off Chance Myers and got a shot past Kansas City goalkeeper Jimmy Nielsen, snapping a 2-2 deadlock.
Ryan Johnson and Darlington Nagbe also scored for the Timbers (3-1-4), who ran their unbeaten streak to six matches with their third win in four contests.
Myers collected both goals for SKC (4-3-2), which has dropped two straight.
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