The incident happened around 10 a.m. in the area of Morton Street and Chelten Avenue.
Police say a shot was heard by those on the bus. There were 25 passengers on board at the time.
The shot may have been fired while the door was open, according to investigators.
Officers were focused on the partition where the bus driver sits.
There were no visible bullet holes, but a bullet fragment was recovered, police said.
There was no immediate word of any injuries.