The robbery was reported at about 8:15 p.m. in the 2400 block of Lombard Street.
A 31-year-old employee was behind the register at the station when a suspect came in and demanded that he open the register, according to police.
The victim opened the register and the suspect took cash from it, then ran to a van that drove away toward Divisadero Street with two other people in it, police said.
The suspects, described as men between 25 and 40 years old, had not been found as of this morning, according to police.
No weapon was seen during the robbery and the employee was not injured.
Anyone with information about the case is encouraged to call the Police Department's anonymous tip line at (415) 575-4444 or to send a tip by text message to TIP411.