A total of three men are being sought for the crime, which happened at the Foodland Market at 5502 Rising Sun Avenue around 2:30 p.m.
Police say two of the men were armed with handguns and moved into the store while the third man appeared to wait by the entrance.
One of the gunmen jumped the counter and pointed a gun at the clerk, demanding the cash register be opened.
Meanwhile, the second gunman went to the rear of the store and forced another employee behind the counter.
After getting $400 in cash, all three suspects fled south on Rising Sun Avenue.
The suspects are described by police as:
Suspect #1: Black male, late teens to 20 years-of-age, wearing a gray hooded sweatshirt with a "North Face" logo in white on the front, black pants, dark colored sneakers, dark colored mask and armed with a handgun.
Suspect #2: Black male, late teens to 20 years-of-age, thin build, wearing a dark colored hooded sweatshirt, dark colored Nike pants with a "Nike" along with the symbol on the left thigh and white vertical stripes on each leg, white mask and armed with a handgun.
Suspect #3: Black male, late teens to 20 years-of-age, medium build, wearing a black hooded sweatshirt, light gray sweat pants, dark colored sneakers, dark colored mask and carrying a green duffel bag.
Anyone with information is urged to contact Philadelphia Police at 215-686-TIPS (8477), text a tip to PPD TIP (773847) or through PhillyPolice.com.