The robbery occurred around 4:15 p.m. at a bank in the 500 block of Castro Street.
A man described as between 38 and 50 years old walked into the bank and passed a note to a teller demanding money, police said.
The teller handed over an undisclosed amount of cash to the suspect, who then fled, according to police. He was still at large this morning.
Anyone with information about the robbery is encouraged to call the Police Department's anonymous tip line at (415) 575-4444 or send a tip by text message to TIP411.