It happened around 9:30 Tuesday night on the 5400 block of Rising Sun Avenue.
Authorities say while inside the store, the suspect went to the counter, pointed the gun at the cashier and demanded money.
The employee complied and the suspect took $211 from the register before fleeing the scene on foot.
The suspect is described as a black male in his 20s, 6'0" tall, 180 pounds with the thin build and medium complexion.
He was last seen wearing a white tee-shirt covering his face, a black knit hat, black jacket with a logo on the front, black Nike pants and black sneakers.
Police say the suspect is considered armed and dangerous.
Anyone with information is asked to contact Philadelphia Police at 215-686-TIPS (8477), text a tip to PPD TIP (773847) or through PhillyPolice.com.