The robbery happened at 4:40 p.m. Wednesday at 2539 Castor Avenue.
Police say the clerk was in the middle of a transaction, and had the cash register open, when the suspect leaned over and grabbed an undetermined amount of money.
The man then fled the store and tried to rob a Dots clothing store, telling an employee he had a gun, but left empty-handed.
The suspect is described by police as:
SUSPECT: Black male, 45-50 years-of-age, 6'4" tall, 240lbs, dark skin, wearing sweat jacket, light colored t-shirt with unknown logo on it, navy blue baseball hat, white pants and navy blue Nike sneakers.
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.