Sixteen-year-old Asir Brown was among a group of about a dozen people attending a barbecue when shots rang out from a passing car.
It happened around 12 a.m. Monday in the 1800 block of South 31st Street.
Police say one of the bullets hit Brown in the back.
Officers performed CPR on the teen until medics arrived and rushed him to the Children's Hospital of Philadelphia.
He was pronounced dead at 12:37 a.m.
Police do not believe Brown was the intended target of the shooter.
Investigators found a dozen spent shell casings at the scene. Bullets pierced several areas of a home, including a screen door and a metal fence.
Investigators were checking surveillance video and talking with witnesses to develop descriptions of the suspect.
They are also looking for the car, described as dark-colored, which fled the scene heading northbound.
Police have not said who the gunman may have been targeting or why.