Mahila court judge G Raja also sentenced Murugan to two years imprisonment for threatening the girl and imposed on him a fine of Rs 5,000.
The sentences will run concurrently.
According to the prosecution, the girl was staying with her mother in the city after her father deserted the family.
Later, the mother developed an illicit relationship with Murugan who started staying with them in the same house.
On August 18, 2012, Murugan had sexually assaulted the girl when she was alone in the house and continued to do so for another three weeks and threatened her with dire consequences if she complained about it.
Based on the complaint, police arrested Murugan.