The bandit held up a TD Bank branch in West Islip on Wednesday.
The man entered the bank at 485 Montauk Highway, and showed one of the tellers a note at noon demanding money. The teller complied with the demands and the man fled on foot.
He was described as white, approximately 5 feet 10 inches tall to 5 feet 11 inches tall with a medium build and in his mid 20s. He was wearing a dark, hooded sweatshirt, dark-colored pants, and dark sneakers with white laces.
Detectives are asking anyone with anyone with information to call anonymously to Crime Stoppers at 1-800-220-TIPS.
All calls will be kept confidential.