Originating in the Early Middle Ages (the name deriving perhaps from the Latinsommum, meaning "top"), Sonnino is mentioned for the first time in a Papal bull from 999. It was held first by the De Sompnino, and was acquired by Onorato I Caetani in 1369. The castle was inhabited by the Caetani d'Aragona until they sold it in 1469 to the Colonna, who were later followed by the Antonelli and the Talani families.
In July 1819 Cardinal Ercole Consalvi, Secretary of State at the Vatican, ordered Sonnino to be razed to the ground as it had become a notorious haunt of the banditi in the Campagna di Roma.
