Kythnos is a Greek island and municipality in the Western Cyclades between Kea and Serifos. It is 56 nautical miles (104 km) from the harbor of Piraeus. The municipality Kythnos is 100.187 km2 (38.68 sq mi) in area and has a coastline of about 100 km (62 mi). It has more than 70 beaches, many of which are still inaccessible by road. Of particular note is the crescent-shaped isthmus of fine sand at Kolona.

Only rarely is the island mentioned by ancient authors. In the Battle of Salamis (480 B.C.), Herodotus records that Kythnos contributed a trireme and a penteconter, and this contribution is commemorated on the base of a golden tripod at Delphi (Herodotus, Bk viii, 46).

Innumerable sources repeat, without providing a citation, that Aristotle praised the government of Kythnos in his "Constitution of Kythnos." Exactly what he wrote is difficult to ascertain, since all of his essays on the constitutions of 158 city-states are lost except for the one on Athens. (Possibly, the origin of the quote is from the 2nd century lexicographer Harpocration.