It happened February 7 just before 2:30 a.m. on the 2200 block of Lincoln Highway.
Police said the suspect first asked to buy gas then demanded the clerk give him $100 from the register.
They said the robber had to stop to gather up some dropped cash but eventually got away.
The suspect is described as a white man, between the age of 20 and 30, approximately 5 feet 7 inches tall, with a brown beard. At the time of the incident, he was said to be wearing black and white sneakers, black pants, a black jacket with stripes going down the sleeves and a red hooded sweatshirt.
He was last seen driving a gray, four-door sedan with silver or chrome rims.
Anyone with information is asked to contact Bensalem police.