The incident happened on March 9 around 9:30 p.m. at the store on the 5400 block of Rising Sun Avenue.
Police say an 18-year-old employee was behind the counter when the male suspect walked into the store with a mask covering his face.
He pointed a handgun at the clerk and tossed over a black and grey backpack stating: "The money isn't yours. Empty the register and put it in the bag."
The robber got away with almost $400 from the cash registers before fleeing south on Rising Sun Avenue.
There were no injuries reported during the robbery.
The suspect is described as a black male, 20 to 25 years-old. He was wearing a black hooded sweatshirt, black skully hat, black half-mask, black pants with a white stripe down the sides and was armed with a handgun.
Anyone with information is asked to call police at 215.686.TIPS (8477) or text a tip to PPD TIP or (773847).
Tips will remain confidential.