The robbery happened around 2:10 p.m. at the Hamilton Marketplace at 5720 Nave Drive.
The suspect told a teller that he had a gun and demanded money, police said. A weapon was not seen and no one was injured, police said.
The suspect is a black man between 20 and 30 years old with a thin build, a mustache and goatee.
He was wearing a black baseball cap with an unknown emblem, a black hooded sweatshirt with a gray design on the front and dark baggy pants.
Anyone with information is asked to call Novato police at (415) 897-1122 or (800) 848-0101.