Trailing Toronto by 10 with 12 second to play in the first half, the Bulls appeared to be on their way to their sixth consecutive loss, but a pair of buckets from Bobby Portis cut the halftime deficit to six.
The Bulls rookie knocked down a jumper then beat the buzzer with a breakaway dunk after a defensive rebound.
Portis has eight points and six rebounds in 11 minutes off the bench while Derrick Rose and Doug McDermott each have 10 points.
DeMar DeRozan has a game-high 18 for the Raptors.
Stats via ESPN