Castiadas is a comune (municipality) in the Province of South Sardinia in the Italian region Sardinia, located about 35 kilometres (22 mi) east of Cagliari. Founded in the 14th century and repopulated in the 19th century after centuries of abandonment, it is part of the Sarrabus-Gerrei historical region.

In 1875 the largest penal colony in Italy was opened in Castiadas, and remained in use until 1956. The construction of the penal colony was part of a larger initiative to repopulate areas of Sardinia that had been abandoned because of malaria and to transform marshy, unproductive land into farmland. The history of the colony is recounted in Il miracolo dei rei, a documentary film by Alessandra Usai.

The area was populated by italian-tunisians, mainly of sicilian descent, immigrated here from Bizerte in 1965.

Castiadas borders the following municipalities: Maracalagonis, Muravera, San Vito, Sinnai, Villasimius.