The robbery was reported around 12:25 p.m. Friday at the SF Fire Credit Union at 565 Buckingham Way.
The suspect, described as a woman in her 30s, entered the bank and handed a teller a note. The teller noticed the woman had a revolver in her right hand, according to police.
The teller, a 23-year-old woman, handed over an undisclosed amount of money to the woman, who then fled the bank and was seen getting into the passenger side of a white 1990s two-door Chevrolet, police said.
The bank robber and her accomplice had not been found as of Monday, according to police.
No one was 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.