Officer Sean Bolton, 33, was shot multiple times during a traffic stop Saturday night by a person who was in the car he pulled over, police said.
Bolton's killer fled, sparking an overnight manhunt, according to Memphis police spokeswoman Karen Rudolph.
A civilian used the officer's radio to notify police about the shooting, she said.
Bolton, who joined the force in October 2010, was taken to hospital in critical condition. He later died.