It happened around 8:35 p.m. Wednesday at the Ho Choi Grocery store on the 2300 block of South 9th Street.
Surveillance video from the scene can be viewed here.
Police say the suspect walked in the store and attempted to grab something from behind the counter.
When the employee stepped in, authorities say the suspect pulled a handgun from his waist and pointed it at the register.
After getting $20 in cash, the suspect fled the store on foot, heading south on 9th Street towards Moyamensing Avenue.
He was last seen wearing a baseball hat with an Air Jordan symbol, sunglasses, a multi-colored button down shirt, dark colored pants and a pink belt.
Anyone with information is asked to call Philadelphia Police at 215-686-TIPS (8477). Tips can also be submitted by sending a text to 773847, or online viaphillypolice.com.