Authorities responded to the 5000 block of Mission Oaks Boulevard at about 2 p.m., but the suspect had already fled the bank with an undisclosed amount of cash.
According to police, the suspect used a demand note during the robbery, but no weapons were seen.
The suspect was described as a white male between the ages of 40 and 50, medium build, unshaven, with gray balding hair. He was wearing a black T-shirt and blue jeans with prescription glasses at the time of the crime.
The FBI is investigating the robbery. Anyone with information is urged to call (310) 477-6565.
A $1,000 reward is being offered by Ventura County Crime Stoppers for information that leads to the arrest of the suspect.