Witnesses told police a large, muscular man wearing a black cowboy hat and a black-and-gray striped, button-up collared shirt entered the bank at 910 Ralston Ave. at about 12:55 p.m.
The man, who was wearing a silver grill in his front teeth, apparently simulated a gun and demanded money from one of the tellers. An actual handgun was never seen, police said.
The suspect then fled into the parking lot after receiving an undisclosed amount of cash. No getaway car was seen.
Belmont police and the FBI are investigating the crime.
The suspect is described as a light-skinned black man with a tightly trimmed beard.
Anyone with information is asked to contact Belmont police at (650) 595-7400.