Ketoy is roughly circular with an area of 73 km² and a diameter of 10 km. Its name is derived from the Ainu language for "skeleton" or "bad".
The island is a complex stratovolcano lined with steep cliffs ranging from 30 metres to 60 metres on the east and south sides with the west and north being taller. The beaches on the island consist of boulders and stones. In the centre of the island is a 1.5 km wide caldera filled by a freshwater caldera lake. The terrain is undulating and steep with numerous hills, rising to two main peaks:
Ketoy is still an active volcano. A major eruption occurred from 1846 to 1847, with the last known eruption in 1960, and August 9, 2013.
Ketoy had no permanent habitation prior to European contact, but was visited in summer by the Ainu tribes from Rasshua for hunting. Claimed by the Empire of Russia, sovereignty was passed to the Empire of Japan per the Treaty of Saint Petersburg along with the rest of the Kuril islands. The island was formerly administered as part of Shimushiro District of Nemuro Subprefecture of Hokkaidō. After World War II, the island came under the control of the Soviet Union, and is now administered as part of the Sakhalin Oblast of the Russian Federation.