Ischia di Castro is a comune (municipality) in the Province of Viterbo in the Italian region Latium, located about 90 km northwest of Rome and about 30 km northwest of Viterbo.
Ischia di Castro borders the following municipalities: Canino, Cellere, Farnese, Manciano, Pitigliano, Valentano.
It takes its name from the nearby Castro, a town destroyed by Papal forces in the 17th century. The town is home to the Ducal Palace, or Rocca, to whose project collaborated Antonio da Sangallo the Younger (also designer's of Castro's walls before its destruction). It was once a palace of the Farnese family.