It happened around 10:40 p.m. Saturday at the 7-Eleven near Atlantic and Grand avenues.
Police say he began to purchase a small item and when the register opened, he allegedly reached in and grabbed some cash.
An employee says there was a brief struggle before the suspect managed to run away.
The worker was not hurt.
Anyone with information regarding this incident is asked to call Nassau County Crime Stoppers at 1-800-244-TIPS.