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 about 0.5 kilometres (0.3 miles) and a highest altitude of 105 metres (344 feet).
The name of the island comes from the Greek for "islet" (small island), νησίς, for which the accusative was nesida.