An ancient Etruscan settlement in the area is certain, due to the several tombs found in and around Piansano.In the 1300s the town belonged to the estate of San Pietro in Tuscia. Then it was fought over by several local families, including the important Bisenzio family and the Vico family until the Pope returned from exile in Avignon.

In 1513 the castle and lands were handed to Cardinal Alexander Farnese (later Pope Paul III) and his son Pier Luigi. As with Arlena di Castro and Tessennano, the area was repopulated in the 16th Century when Cardinal Alexander Farnese brought in families from Umbria to work the land.
In 1537 Piansano was incorporated in the Ducato di Castro, but returned to the Church in 1649 after the destruction of Castro. In 1790 it was handed to Count Alessandro Cadelli and then in 1808 to the Polish prince Stanislaw Poniatowski. During the 19th Century Piansano became famous as one of the hiding places of the brigand Tiburzi.