The man walked into the Bank of America in the Rossmoor Shopping Center at 1200 Rossmoor Parkway at about 5 p.m. and passed a note to a teller demanding money, police said.
He ran away after receiving cash and a witness said someone with a similar description got into a Honda with tinted windows and mirrors, according to police.
The man was described as a white man in his mid-20s standing about 6 feet tall with a thin to medium build and a light brown goatee. He was wearing a long-sleeved striped shirt with a dark collar and a camouflage baseball hat.
He was not seen with any weapons, police said.
Anyone with information about the robbery has been asked to call Walnut Creek police at (925) 943-5844.