Syros (Greek: Σύρος), or Siros or Syra is a Greek island in the Cyclades, in the Aegean Sea. It is located 78 nautical miles (144 km) south-east of Athens. The area of the island is 83.6 km2 (32 sq mi) and it has 21,507 inhabitants (2011 census).

The largest towns are Ermoupoli, Ano Syros, and Vari (el). Ermoupoli is the capital of the island and of the Cyclades. It has always been a significant port town, and during the 19th century it was even more significant than Piraeus. Other villages are Galissas, Foinikas, Pagos, Manna, Kini and Poseidonia.