Maria Monaco, 47, was found huddled in the corner of her squalid bedroom surrounded by filth and excrement.
The case has shocking similarities to that of Elisabeth Fritzl, the Austrian woman kept in a cellar and regularly raped by father Josef for 24 years.
Monaco was found by police at Santa Maria Capua Vetere, a small town just outside Naples, after an anonymous tip-off.
Her brother and sister are in custody accused of detaining her.
Footage on Italian TV showed a squalid stained single mattress in her bedroom, dirt and grime on the floor and walls and a bathroom covered in filth.
Police said Monaco, who suffers from psychological problems, was made a prisoner shortly after falling pregnant with her child, now a 17-year-old student.
Her mother Anna Rosa Golino, 80, brother Prisco, 44, a farmer, and sister Michelina, 54, a primary school teacher, took action after she had an affair with a mystery man and got pregnant.
Capt Carmine Rosciano, of Santa Maria Capua Vetere police, said: "We received an anonymous tip-off that a woman was being held and we raided the family home on rural land on the outskirts of the town.
"Maria was found in her locked bedroom in a confused state and surrounded with filth and excrement she was living in squalor."
She is now in hospital in Naples being treated for her trauma, while her mother was bailed.
Police were also questioning Monaco's 17-year-old son, who they said was living with his grandmother and was "fully aware of the situation".
Attempts were also being made to track the father of the boy, who police refused to name.
They said it was proving difficult to question Monaco because of her psychological state.