The robbery happened at 2:10 p.m. Tuesday at the Cricket Wireless Store at 5547 Baltimore Avenue in Kingsessing.
Investigators say the man waited for 30 minutes until customers in the store left.
Then, police say, the suspect pulled out a handgun and ordered a worker to hand over money from the register.
Police say the suspect took the cash along with two cell phones and fled the store on foot.
The suspect is described as a black male, about 20 years old standing six feet tall. He had a medium build and a beard. He wore a black hooded sweatshirt with a Puma logo with white letters.
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.