Investigators say the 19-year-old victim and his girlfriend were delivering drugs to a man in the area of San Ramon and 6th Street.
When the suspect went up to the car, he pulled out a gun and tried to rob the pair.
The 19-year-old was shot at least once in the upper body while trying to drive away. Police say he was taken to the hospital in critical condition.
The suspect left on foot, and investigators are searching neighboring streets. Officers say he was wearing a dark-colored hooded sweatshirt and red pants.
Police have the street blocked off while they continue their investigation.