According to police, the victim's mother divorced her first husband and married to the accused in 2008.
The accused began touching the girl inappropriately in 2010 and then started sexually assaulting her at their residence in Andheri, police said.
The school-going girl was threatened of dire consequences if she revealed anything to her mother, they added.
The victim had so far kept mum. However, on Sunday, she finally told about it to her mother, who took up the matter with the Juhu police.
The accused was arrested on Monday and produced before a local court which remanded him in police custody till May 13, police said. Further probe in the case is on.