Kythnos belongs to the northwestern Cyclades and is located between Kea and Serifos.
The island extends over a surface of 99.4km2 and the total length of its coastline is 111km.
The island’s highest peak (at 356m) is on Mount Kakovolo in the north. There is also a mountainous area to the south, whose highest peak is Profitis Ilias (326m). The settlement of Chora is located west of this area.
The coastline of the island has sharp curves and many beaches. The island's soil contains shale and its subsoil is relatively rich in iron and copper deposits, the existence of which has been known since the Antiquity.
Kythnos has a temperate marine climate. The island is considered to be one of the driest areas of the Cyclades. Due to the scarce precipitation, vegetation is limited to garrigue and areas of halophytic vegetation, with Jerusalem sage, thorny burnet and thyme being the most common species on the island.
The island’s diverse avifauna is similar to that of the rest of the Cyclades. The Common Kestrel (Falco tinnunculus), the Partridge, the European Swallow, the Carrion Cow, the House Sparrow, the Common Blackbird and the Wild Dove are among the bird species living on Kythnos.
The Atheras mound, the promontory of Kefalos and the coastal zone of the island have been included in the NATURA 2000 European Network of Protected Areas as Sites of Community importance (SCI).
The existence of many windmills both in towns and further inland shows that agriculture played an important role in the local economy for centuries, regardless of the infertile soil of the island. Cereals were the main crop on Kythnos and the flour produced from them was the principal export product of the island.