It happened at the TD Bank branch on Horseblock Road at about 5:50 p.m. Thursday.
The robber handed a teller a note that contained a threat and demanded cash.
The teller gave the man cash from the drawer and he ran out of the bank and across Horseblock Road.
The man was last seen running through a shopping plaza parking lot located across the street from the bank on North Ocean Avenue.
The suspect is described as a white man in his late 30's or early 40's, about 6 feet tall, and wearing a tan short-sleeve polo shirt and a classic blue Yankee hat with a white logo.
Detectives are asking anyone with information on this case to call the Major Case Unit at 631-852-6295 or anonymously to Crime Stoppers at 1-800-220-TIPS.
All calls will be kept confidential.