The robbery was reported at about 11:05 p.m. Tuesday in the 2800 block of Mission Street near 24th Street.
The suspect, a man in his mid 20s, entered the business through a back door and pointed a black semi-automatic handgun at a 41-year-old woman inside, according to police.
The man took cash and a cellphone from the victim and then ran away. He had not been found as of this morning, police said. The woman was not injured in the robbery.
Anyone with information about the case is encouraged to call the Police Department's anonymous tip line at (415) 575-4444 or send a tip by text message to TIP411.