The incident took place on Sunday, 3 July when the girl was playing outside her residence, they said.
The Laxmi Nagar police station received information about the alleged sexual assault on Sunday morning.
Deputy Commissioner of Police (East) Priyanka Kashyap said a preliminary enquiry revealed that when the victim was playing outside her house, an unidentified man approached her, took her to a secluded place, and sexually assaulted her.
The girl subsequently narrated the incident to her parents, following which a complaint was lodged, police said, adding that the victim was medically examined and counselled.
"We have registered a case under the appropriate sections of the Indian Penal Code and the Protection of Children from Sexual Offences Act and an investigation has been taken up," Kashyap said.
Police have zeroed in on the suspect and efforts are on to nab him, the DCP added.
(Published in arrangement with PTI.)