Nisida is a volcanic islet of the Flegrean Islands archipelago, in southern Italy. It lies at a very short distance from Cape Posillipo, just north of Naples; it is now connected to the mainland by a stone bridge. The islet is almost circular, with a flooded crater forming the bay of Porto Paone on the southwest coast. It has a diameter of c. 0.5 km and a highest altitude of 105 m.
In ancient times Lucius Licinius Lucullus built a villa on Nisida, and also Marcus Iunius Brutus had a partician residence here. Here, the latter's wife Porcia, the daughter of Cato Uticensis, committed suicide. The claim is made that some of archaeological remains on Nisida are, indeed, those of the villa of Brutus, and that here is where the conspiracy to assassinate Julius Caesar was hatched. In the 16th century a castle was built, which was subsequently a fief of the Macedonio family.