The 15-year-old boy took the victim to a secluded place on the night of November 22 and sexually assaulted her near the Pragati Nagar slum colony in Nalla Sopara area of Thane district, assistant police inspector A P Nigavkar said.
The complaint was registered by the victim's family yesterday after which the accused was nabbed.
He has been charged under the IPC and also under the Protection of Children from Sexual Offences Act, 2012, police said.
The boy was produced yesterday before a magistrate, who ordered him to be taken to the Bhiwandi remand home, they added.