The robbery was reported at 9:44 a.m. at a bank in the 200 block of California Street.
A suspect, described as a white man in his early to mid-50s, walked into the building and announced he was robbing the bank, according to police.
Two tellers handed the robber cash, and he then fled and had not been found as of this morning, police said.
No one was injured and no weapon was seen.
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.