Police say the man entered the Bank of America on Long Beach Road Friday morning and passed a note to an employee demanding money.
He threatened the use of a gun which was in a holster on his right hip.
According to detectives, he then grabbed the note from the employee and fled the bank with no proceeds.
There were two customers and six employees in the bank at the time, but no one was hurt.
The suspect is described as being 40-45 years of age, 5'10" tall, thin build, wearing a long sleeve white shirt, with a yellow square patch on the left breast pocket and a white and blue patch on his left shoulder.
He was wearing black pants, a black baseball cap with white and red letters and dark sunglasses.
Anyone with information is asked to contact Nassau County Crime Stoppers at 1-800- 244-TIPS.