The robbery happened about 9:50 p.m. Monday at 1144 Christiana Road.
The robber confronted a 50-year-old clerk and demanded cash from the register. He turned over an undisclosed amount and the robber fled on foot and was last seen running north on Greentree Road.
The robber was described as white man about 30-40 years old and 5-feet-9 to 5-feet 10, weighing 160-180 pounds. He was wearing a blue hooded sweatshirt, blue jeans, a black mask and black shoes.
Anyone with information is asked to call troopers at 302-365-8566.