On the second play on overtime, Kansas City quarterback Matt Cassel sent a wobbling pass into the left flat where Timmons picked it off at the Chiefs' 28-yard line, returning it five yards.
The Steelers (6-3) elected to kick immediately, and Suisham nailed the 23-yarder for the victory, which kept Pittsburgh unbeaten at home.
The Chiefs (1-8) had forced overtime when Ryan Succop hit a 46-yard field goal on the last play of regulation.
Ben Roethlisberger completed 9-of-18 passes for 84 yards and a score before leaving in the third quarter with a right shoulder injury. Byron Leftwich entered and went 7-of-14 through the air for 73 yards the rest of the way.
Cassel finished 11-of-26 for 154 yards and an interception in the loss.
Jamaal Charles racked up 100 yards on 23 carries with a score for Kansas City.