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. Population was 2,299 in 2021. Ios is part of the Thira regional unit.