Surveillance photos from the FBI show the bandit robbing a Bank of America branch at about 5:40 p.m. Friday in San Luis Obispo.
He pointed a weapon at the teller and got away with an undisclosed amount of cash.
The "Geezer Bandit" is described as Caucasian, between 60 and 70 years old, often wearing a baseball cap and dark slacks.
He started robbing banks two years ago, mostly in the San Diego area.
Anyone with information is urged to call the FBI.