It happened around 2:30 a.m. Sunday in the 1400 block of Levick Street.
Investigators say the 10-year-old victim awoke to an unknown man indecently assaulting her.
She screamed, alerting her 74-year-old babysitter, Julie Etienne, who was sleeping in the same room.
Police say Etienne sprang into action, grabbing the man. She says the man pushed her, and she stumbled down some stairs.
As Etienne recovers, police say the intruder assaults the 10-year-old girl. Etienne then regains her footing, police say, and goes after the intruder again.
At one point in the struggle, Etienne bites the intruder on the arm. The man eventually gets away.
According to investigators, the man got into the house through an unlocked rear door.
The suspect is described as a black man in his late teens or early 20's. He was six feet tall with a thin build and he had freckles, acne scars or spots on his face. He was wearing a gray shirt and blue jeans.
Anyone with information on this crime is urged to contact Philadelphia Police at 215-686-TIPS.