The incident took place yesterday and the 15-year-old accused was arrested the same day following a complaint, Police Inspector S R Nagarkar of Mumbra police station said.
According to Nagarkar, the accused and the victim lived in the the same building. Yesterday, the victim had gone to the boy's house to return a book when he asked her to come inside and then sexually assaulted her.
The victim later told about the incident to her family members who immediately lodged a complaint at Mumbra police station and the boy was arrested.
The accused has been booked under section 376 of the IPC and sections of Protection of Children from Sexual Offences Act, 2012, the police said. The arrested accused has been sent to the juvenile court, police said.