Ios, Io or Nio is a Greek island in the Cyclades group in the Aegean Sea. Ios is a hilly island with cliffs down to the sea on most sides. It is situated halfway between Naxos and Santorini. It is about 18 kilometres long and 10 kilometres wide, with an area of 109.024 square kilometres (42.094 sq mi). Population was 2,299 in 2021. Ios is part of the Thira regional unit.

The Cyclades are an island group in the Aegean Sea, southeast of mainland Greece and a former administrative prefecture of Greece. They are one of the island groups which constitute the Aegean archipelago. The name refers to the archipelago forming a circle around the sacred island of Delos. The largest island of the Cyclades is Naxos, however the most populated is Syros.