The shooting was reported at 11:59 a.m. in front of a McDonald's restaurant at 24th and Mission streets, San Francisco police spokesman Officer Albie Esparza said.
The 19-year-old victim was taken to a hospital to be treated for his injury, which is not believed to be life-threatening, Esparza said.
Two suspects are believed to have been involved in the shooting and were seen running into the BART station, according to Esparza.
BART police were notified and the train tunnel was briefly shut down while authorities searched the station, but the suspects were not found, Esparza said.
They are described as a Hispanic and black male, one of whom was wearing a white hat and blue jacket, Esparza said.
The shooting is the latest act of violence in the Mission District, which has had three homicides in the last eight days.
The violence prompted police to increase patrols in the area.
Anyone with information about today's shooting is encouraged to call the Police Department's anonymous tip line at (415) 575-4444 or send a tip by text message to TIP411.