Kythnos has been inhabited for thousands of years: at Maroula, near Loutra, one of the oldest settlements in the Aegean came to light, dating to the Mesolithic period (around the 9th millennium BC). According to mythology, the island took its name from Kythnos, leader of the Dryopes, the ancient tribe that settled here — hence also the name Dryopida.
In antiquity the capital was ancient Kythnos (Vryokastro) on the west coast, a thriving city with temples and an agora that was continuously inhabited from the Geometric to the Roman era. The island took part in the Athenian League and was known for its mineral wealth — iron and copper — while its thermal springs had already made it famous.
In medieval times, after the Fourth Crusade, Kythnos passed to the Venetians and became a fiefdom of the Gozzadini family, who governed the island for centuries from the Castle of Oria at the northern tip. The Gozzadini held power unusually late — until the early 17th century — leaving their mark, such as the coat of arms at Agios Savvas in Chora.
Under Ottoman rule from 1617, the island retained its agricultural and seafaring character, with residents moving from the Castle to the present-day villages, Chora and Dryopida. After joining the Greek state, during the 19th and 20th centuries mining activity flourished (iron ore extraction at Katafyki), while in 1857 the thermal baths were officially organized. Today Kythnos relies on gentle tourism, keeping its authentic identity intact.