After Portland knocked it out of the Open Cup on Tuesday, KC responded by denting the Timbers' playoff hopes.
The Timbers went close early on, with a lovely Darlington Nagbe-Fenando Adi combination seeing the latter put the ball in the back of the net, but the assistant referee judged the Nigerian striker to be just a hair offside.
Lawrence Olum scored the game's only goal in the 24th minute. A Seth Sinovic long throw was headed down by Kevin Ellis, and Olum turned on the ball and shot first-time past Donovan Ricketts.
The Timbers are now two points off the playoff pace but are tied for most games played in MLS and have a 4-5-8 record. Kansas City goes to first in the Eastern Conference with its 7-5-4 record, ahead of D.C. United on goal difference, although United has a game in hand.