Officers say the shooting happened on the 6600 block of Bustleton Avenue in Northeast Philadelphia just before 6:30 p.m. Sunday
Investigators say the two teens knew each other and got into an altercation at a nearby Wawa.
The 17-year-old then carjacked a vehicle and drove around looking for the victim.
Police say when he found the victim walking down Bustleton Avenue with a friend, he got out of the car and opened fire.
The 19-year-old was shot once in the head. He has been identified as Kairo Lessa-Neves from the 6900 block of Revere Street
He was transported to Albert Einstein Medical Center where he was later pronounced dead.
Police say they know the identity of the 17-year-old suspect and are searching for him.
The suspect's name has not been released.