It happened around 8 p.m. in the city's Hunting Park section.
Police said an 11-month-old was shot four times: once in the head, chest and twice in the backside while inside a vehicle.
Philadelphia police are providing conflicting information on if the baby is a boy or a girl.
The child's stepmother was in the area and heard shots fired, police said.
Police said the child's stepmother continued driving and made it a few blocks from the area of the shooting before she reportedly realized that her car and that the baby had been shot.
Investigators are continuing to canvass the scene looking for evidence of a shooting. So far, they've said they haven't found any bullet casings.
WPVI reported that car which police say was shot up at least five times.
The baby is listed in extremely critical condition.
No arrests have been made in either case.