Castello di Nisida, Renaissance castle on Nisida island, Naples, Italy.

Castello di Nisida stands on the south-eastern side of the islet of Nisida in the Flegree islands, above the inlet of Porto Paone. Its old crumbling tower was demolished and in 1933 the rooms beneath were converted into a cell block with an infirmary and quarantine rooms; administrative offices and staff quarters occupy the upper floors. In front of the entrance a wide avenue leads past pavilions for dormitory, school, refectory, flowerbeds and playing fields, with a large sports field on a lower level.

The castle probably dates to the late Angevin period and was reworked from the 16th century as a stronghold in Pedro de Toledo's defensive system from Baia to Sperone, after raids by Barbarossa on Calabria, Ischia and Procida. In 1656, during the plague, viceroy Antonio Álvarez de Toledo y Beaumont made it a lazaretto. Under the Bourbons it held political prisoners; in 1851 Gladstone denounced their conditions, and after unification it detained former Bourbon officials.

The castle and the whole islet belonged to the Neapolitan Macedonio family, dukes of Grottolella. In 1851 the count of Gladstone denounced the inhuman conditions of the political prisoners held here under the Bourbons. After unification the Savoy purge turned it into a jail for former Bourbon officials.

The fortress currently serves as a youth detention facility under the Italian Ministry of Justice and is not open to the public. Visitors cannot access the building or its grounds due to its institutional status.

In 1851 the British statesman William Ewart Gladstone denounced the inhumane conditions of the prisoners held in the castle. After Italian unification the same cells received former Bourbon officials purged by the Savoy administration. Since 1933 the south-eastern side has housed the juvenile prison, whose art workshop is open to residents of the neighbourhood.