Police say the man entered the bank, on Larkfield Road at approximately 5:30 p.m. and pulled out an automatic handgun and verbally demanded cash from a teller.
The teller gave the man cash from the drawer, and the suspect fled on foot, north through the parking lot.
No one was injured.
The suspect is described as white, between 50 and 60-years-old, between 5 feet 8 inches tall and 5 feet 9 inches tall, with a stocky build.
He was wearing a short-sleeve shirt with a dark t-shirt underneath, a dark hat with no logo, blue jeans, white sneakers and was carrying a dark backpack.
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.