It happened around 9:30 p.m. at the Hancock Playground at Master and N. Mascher streets.
According to Chief Inspector Scott Small, a man walked onto the playground and fired at least five gunshots while people were playing basketball and kickball.
"We know that there was a large amount of people at this recreation center, including teenagers, juveniles, young adults and other adults that were on the property at the time," said Small.
A 35-year-old woman was shot in the back. She was rushed to the hospital in critical condition.
A 16-year-old boy suffered a gunshot wound to the shoulder. He is expected to survive.
Small said it's unclear if the teenager and woman were involved in any of the recreational activities at the time of the shooting. It's also not clear if they are related to each other.
One of the bullets did enter a nearby residence, but no one was injured.
According to Small, people of all ages - including the very young - saw the shooting unfold.
"In the case here, we had a large crowd of people enjoying a beautiful night and playing organized sports, that's something we want them to do. That's why we need the public, the community, to tell us anything they know so we can apprehend the responsible person or persons and take them into custody and get them off the streets," Small said.
No arrests have been made. Police believe the gunman got away in a dark-colored vehicle.
Investigators aren't sure whether the victims knew the shooter.
Anyone with information is asked to call police at 215-686-TIPS.