Surveillance video shows the man lurking outside of a home along the 1500 block of North 15th Street on Monday night.
Police say a 20-year-old woman was getting dressed when she saw the man looking in her window and taking pictures.
After being spotted, the man ran off in an unknown direction.
The suspect is described as a white male, about 20 years old, 6 feet 2 inches tall, with a thin build and brown hair.
He was seen wearing a black baseball cap, and a black North Face jacket.
Anyone with information is asked to contact police at 215.686.TIPS (8477).