After a goalless draw in the first leg, the two sides put on a show at Sporting KC's Children's Mercy Park in a thrilling encounter that ended with the visitors escaping with a 3-2 victory.
Sporting KC broke the 110-minute deadlock between the two sides when Daniel Salloi opened the scoring in the first half.
After the break, midfielder Sebastian Blanco unleashed an unstoppable shot from distance to give the Timbers a valuable away goal. Minutes later, Diego Valeri headed the ball into the back of the net to make it 2-1 for the Timbers.
Gerso Fernandes gave the home supporters hope when he made it 2-2 with a strike near the penalty spot.
But Sporting KC's hopes of reaching the MLS Cup final for the first time since 2013 were erased when Valeri added his second goal in the 99th minute.
The Timbers - winners of the 2015 final - will attempt to win their second MLS Cup when they take on Atlanta United at the Mercedes-Benz Stadium on Dec. 8.