Arlena di Castro is a comune (municipality) in the Province of Viterbo in the Italian region Latium, located about 80 km northwest of Rome and about 15 km west of Viterbo.
Arlena di Castro borders the following municipalities: Cellere, Piansano, Tessennano, Tuscania.
The town was founded in the early 16th century by Alessandro Farnese, the future Pope Paul III, to house refugees from Allerona.