The teenager has been identified, but his name hasn't been released because the shooting could be gang-related, police Officer Jose Garcia said.
The shooting happened a few minutes after noon in the driveway of the McDonald's at 898 S. Bascom Ave., across from the Santa Clara Valley Medical Center.
Garcia said it appears that the shooter walked up to the victim and shot him once at close range then ran north on South Bascom Avenue.
The victim was taken to Santa Clara Valley Medical Center and put on life support sometime around 5 p.m.
Police are still searching for the shooter, who is described a Hispanic or white male in his late teens or early 20s with a light complexion.
Garcia said witnesses gave conflicting statements about the suspect's clothing, so police aren't releasing any other details about his description.
Police have yet to recover the firearm and are assuming that the suspect is still armed and dangerous.