Valentano has an excellent hillside location from which they control the valleys below. Its origins are very old, as evidenced by the finds from the Bronze Age around Lake Mezzano. Maybe the name comes from the ancient Etruscan town of Varetum, never identified, or from a Valente noble who had properties in the area.
The first news of the medieval village date back to 813 DC in the register of the Abbey of Farfa. In 1193 it became part of the domains of Viterbo, in 1212 it went to Orvieto and among the possessions of the Holy See during the papacy of Pope Urban IV.
It was destroyed in 1328 by the armies of the Emperor Louis of Bavaria and after the transfer of the popes in Avignon, suffered the assaults of the Prefects of Vico and other families.
In 1357 Valentano was retaken by Cardinal Albornoz and attached again to the Papal States, and in 1368 Pope Urban V gave it to Ranuccio and Puccio Farnese. With this family it has experienced a period of prosperity interrupted in 1649 with the Castro war between the Farnese and the Apostolic Chamber. The Farnese abandoned Vallerano which returned to the Church.
Valentano gave birth to the illustrious mathematician Paolo Ruffini (1765-1822).