The incident happened around 1:25 p.m. Tuesday at the bank on the 1300 block of 4th Street.
According to witnesses, the suspect entered the bank and handed a note to a teller, demanding money. After getting the cash, he fled the scene on foot.
The suspect was described as a man in his mid-40s. He was about 5 feet 8 inches tall with a "big belly," officials said.
Anyone with information was asked to contact Santa Monica police at (310) 458-8495.