It happened about 4:15 p.m. Sunday at the Del One Federal Credit Union on Christiana Road.
Troopers say a man had withdrawn money from the ATM and was driving away when a car pulled in front of his vehicle.
Police say two men pulled the victim from his car and demanded money. The victim gave the robbers cash and one hit him in the face with a handgun. The two suspects then drove off.
The victim was treated and released at a local hospital.