The robbery happened at Cosi located at 15th and Locust.
It happened around 7:00 a.m. just after opening.
The suspect fled with about $1,500.
There were no reports of injuries.
The suspect is described as a black male in his early 20's, standing about 5'10" to 5'11" tall, wearing a dark hooded sweatshirt and white hospital mask.
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.