"I did not know I was pregnant. I only felt a stomach pain," the nun was quoted as saying at the hospital.
The 31-year-old mother and her baby boy, who weighs 3.5 kilogrammes (nine pounds), are doing
well and people at the hospital have begun collecting clothes and donations for the mother and her child.
The nun belongs to the "Little Disciples of Jesus" convent in Campomoro near Rieti, which manages an old people's home and reports said she would keep the baby.
The nun had named her baby Francesco (Francis) -- also the pope's chosen title and one of the most popular names in Italy, where St Francis of Assisi is the much-loved national patron saint.
Her fellow nuns were quoted saying they were "very surprised" while the mayor of Rieti, Simone Petrangeli has appealed to the public and media to respect the woman's privacy.
