Nisyros (Greek: Νίσυρος; also transliterated Nissiros) is a volcanic Greek island located in the Aegean Sea. It is part of the Dodecanese group of islands, and lies between Kos and Tilos. Its shape is approximately round, with a diameter about 8 km, and its area is 41.6 km². Several other islets are found in the vicinity of Nisyros, the largest of which is Gyali. The island has a mountainous interior and in its center several craters are found. Its coasts are generally rocky or pebbled, but there are also a few sandy beaches.

The volcano is currently inactive, but fumaroles are found at the craters. Its latest eruption was on 1871, but earthquakes are not infrequent.

According to Greek mythology, the island was formed when Poseidon cut a part of Kos and threw it on the giant Polyvotis to stop him from escaping. The ancient name of the Nisyros was Porphyris.

The main town and port of the island is Mandraki. Other villages are Paloi, Nikeia and Emporios. According to a 2001 census, the resident population is 938, although in summer it is augmented by many tourists as well as expatriate Nisyrians who visit the island for their vacations. Tourism is not so heavily developed as in other Greek islands. On Gyali there are deposits of perlite and pumice which provide much of the wealth of the island. The island used to be self-sufficient, and many crops were grown on its terraced slopes. Today, though, they are cultivated on a smaller scale.

There is a health spa with sulphurous waters that used to be world-famous in the first decades of the 20th century.

A traditional product of Nisyros is soumada, a non-alcoholic almond-flavoured drink.

The patron saint of the island is Saint Nikitas. Many Orthodox Christian churches are found on the island, as well as four monasteries which are not inhabited by monks today, although various celebrations take place in them. The largest monastery is the one of Panagia Spiliani (Blessed Virgin Mary of the cave) at Mandraki. It is built beside the medieval castle erected by the Knights Hospitaller who conquered the island on 1315.

Ancient walls, dating from the 5th century BC, part of the acropolis of the island, are found near Mandraki.

There is a heliport.