The shooting was reported shortly before 1:40 p.m. in the first block of Garrison Avenue.
Officers responded and found the teen lying on the ground with three or four gunshot wounds to his torso, police spokesman Officer Carlos Manfredi said.
The victim was taken to San Francisco General Hospital to be treated for his injuries, which are considered life-threatening.
Witnesses told police the suspect got on a Muni 9-San Bruno bus and got into a verbal confrontation with the victim, who was riding the bus at the time, Manfredi said.
Then, as the victim exited the rear of the bus, the suspect opened fire from the bottom step of the rear bus doors and then ran away. He was last seen running north on Garrison Avenue, according to Manfredi.
The shooter is described as a black man between 20 and 25 years old who is about 6 feet tall, has dark skin, short-cropped hair, and a small moustache. He was wearing a black hooded sweatshirt, white T-shirt and saggy light blue jeans, Manfredi said.
Muni officials handed over the surveillance video from the bus to police, Muni spokesman Paul Rose said.
The Police Department's gang task force is reviewing the footage and is leading the investigation into the shooting. Anyone with information about the case is encouraged to call the gang task force at (415) 553-1401.
People wishing to remain anonymous can call the department's tip line at (415) 575-4444 or send a tip by text message to TIP411.