The man has been arrested, they said.
According to police, the incident took place on Sunday afternoon when the girl was alone with her stepfather at their residence in West Delhi's Punjabi Bagh area.
"The man raped the girl when her mother had gone to market to buy some groceries. When she returned, the victim narrated her ordeal to her, following which she filed a complaint against her husband," said a police official.
The medical examination of the girl confirmed rape.
In a similar incident, a man was arrested for molesting an eight-year-old girl after he lured her to his residence in West Delhi's Kirti Nagar area on Sunday afternoon.