Vetralla is a town and comune in the province of Viterbo, in central Italy, 11 kilometres (7 mi) south of that city, located on a shoulder of Monte Fogliano.

The site was depopulated in the later Empire, when a smaller population retreated to the present strategic position commanding the valley, where it remained exposed to attack, in spite of the imposing walls that encircled it.

The little fortress had been incorporated into the Papal States from their historic beginnings with the Lombard king Liutprand's Donation of Sutri (728) to Pope Gregory II, but the lords of Viterbo held it from 1110 to 1134.

By 1145, Pope Eugene III was installed at Vetralla, safely removed from the violence and party strife of Rome; from here he issued his bull Quantum praedecessores, calling for the Second Crusade.

To this day, in the annual ceremonial Sposalizio dell'albero ("marriage of the tree"), the mayor of Vetralla reaffirms the town's rights of possession on Monte Fogliano.