Kasos (ˈkɑːsoʊs; Κάσος, ˈka̠so̞s), also Casos, is a Greek island municipality in the Dodecanese. It is the southernmost island in the Aegean Sea, and is part of the Karpathos regional unit. The capital of the island is Fri. , its population was 1,224.
There are several references to the island in the works of ancient authors, including as Amphe (Ἄμφη), Astrabe (Άστράβη), and Achni (Άχνη). Concerning Kasos (Κάσος), Samuel Bochart (1674) and Victor Bérard (1902) suggested that it could derive from the Canaanite word kas , and that it is a doublet with Greek Achni (Άχνη). The island is also known in Italian as Caso and in Turkish as Kaşut or Çoban (چوبان).
Kasos lies southwest of Karpathos, and east of Crete. The island lies within the subtropical zone, being at 35oN latitude.
Adjacent to the island is the Strait of Kasos, through which some of the Modified Atlantic Water enters the Sea of Crete. Its shape is elliptic and resembles that of Rhodes. The main island has a surface of , and it is long and wide. It is very mountainous, with its highest mount being Mt. Prionas, which is high. There is fresh water on the island. Lawrence Durrell is rather disparaging about the island, begrudging it a mere 22 words in a brief comparison to Karpathos, calling it a "smaller, stonier version of the same sort of thing."
The Florentine monk and geographer Christoforo Buondelmonti, around 1415, explored Karpathos and makes a reference to seeing Kasos in the distance. His map of Karpathos seems to include a schematic view of Kasos to the west. In 1687, the English merchant Bernard Randolph referred to the island as Cassio or Casos, and wrote the following about it:
Tho 'tis but a small Island, yet it abounds in all things necessary for life. The middle of the Island is almost a plain, well planted with Olive trees, and Vine-yards; with all sorts of Fruits. To the West are five small Islands, amongst which is very good riding for ships.