The robbery was reported shortly after 5 p.m. near Holyoke and Woolsey streets.
The boy was walking along the sidewalk when he encountered a man next to a car, police spokeswoman Officer Tracy Turner said.
The man demanded the boy's cellphone and wallet, and the boy was "so scared he didn't do anything," prompting the suspect to rifle through the boy's pockets and take his property, Turner said.
The suspect, described as between 20 and 30 years old, then got into a red four-door sedan occupied by three other people and the car drove away, according to Turner.
The boy was not injured in the robbery.
Anyone with information about the case is asked to call the Police Department's anonymous tip line at (415) 575-4444 or send a tip by text message to TIP411 with "SFPD" in the message.