The Duchy of Castro was a fiefdom in central Italy formed in 1537 from a small strip of land on what is now Lazio's border with Tuscany, centred on Castro, a fortified city on a tufa cliff overlooking the Fiora River which was its capital and ducal residence. While technically a vassal state of the Papal States, it enjoyed de facto independence under the rule of the House of Farnese until 1649, when it was subsumed back into the Papal States and administered by the House of Stampa di Ferentino.
It was created a duchy by Pope Paul III (1534–1549) in the bull Videlicet immeriti on 31 October 1537, with his son Pier Luigi Farnese and his firstborn male heirs as its dukes. It lasted approximately 112 years and was eclipsed by the Farnese's possessions in Parma. It stretched from the Tyrrhenian Sea to the Lago di Bolsena, in the strip of land bounded by the river Marta and the river Fiora, stretching back to the Olpeta stream and the lago di Mezzano, from which the Olpeta flows. The duchy of Latera and county of Ronciglione were annexed to it.
