The nun, who came to Italy from South America, belonged to an order called the Missionary Sisters of the Love of Christ in Macerata, in the Italian region of Le Marche.
She said she had no idea she was pregnant.
She was taken to hospital by her sisters, where doctors told her she was pregnant, according to Italian newspaper Il Corriere Adriatico.
The baby is said to be healthy and the nun's convent has expressed an interest in taking care of him.
The nun arrived at the convent from Bolivia in June and it's believed she was already pregnant. She has now been discharged from hospital.
The story has echoes of the nun who gave birth to a baby boy in the small central Italian city of Rieti last summer.
The 31-year-old woman complained of stomach pains and was rushed to hospital but when she arrived doctors told her she was expecting.
The woman, who was originally from El Salvador, named her child Francesco (Francis) after the current Pope.
According to the Ansa news agency, she said: "I did not know I was pregnant. I only felt a stomach pain."The nun lived at the Little Disciples of Jesus convent near Rieti, which manages an old people's home.
At the time, local pastor Don Fabrizio Borrello said the new mother planned to take care of the baby.
He said: "I guess she's telling the truth when she says she arrived at the hospital unaware of the pregnancy."