Kythnos, commonly referred to as Thermia (Θερμιά), is a Greek island and municipality located in the Western Cyclades. It is situated between the islands of Kea and Serifos, at a distance of approximately 56 nautical miles (104 km) from the harbor of Piraeus in Athens.
The municipality covers an area of 100.187 km2 (38.68 sq mi) and features a coastline of about 100 km (62 mi). The island's highest point is Mount Kakovolo, which reaches an elevation of 365 meters.