It happened around 4:30 Thursday afternoon on the 5600 block of Germantown Avenue.
Surveillance video of the robbery can be viewed here.
Police say the suspect entered the store, walked behind the counter and placed a 10 inch knife to the employee's back.
According to authorities, the woman said "Don't get in my way," and took an unknown amount of money from the register and a number of in-active cell phones from the counter.
The suspect placed the items in a large black bag with a reptile pattern on the side and fled south down Germantown Avenue on foot.
The woman is described as a black female, 5'2" tall, 140 pounds. She was last seen wearing dark Muslim attire and a silver belt.
Anyone with information is asked to contact Philadelphia Police at 215-686-TIPS (8477) or text a tip to PPD TIP or 773847.