The incident took place yesterday at a bungalow in the hill station Panchgani in the district, they said, adding the boy was apprehended and produced in the juvenile court today.
The 8-year-old victim's body was recovered in a water tank of the bungalow where the boy's mother is a caretaker, they said.
"He has confessed to raping and killing the girl, a second standard student, all within a span of 20 minutes," a senior police official claimed.
The boy is a close relative of the 8-year-old victim.