Onekotan Island (Онекотан), also known as Onekotan To, is an island located near the northern end of the Kurils (). Onekotan is a volcanic island, part of the "Ring of Fire" that encircles the Pacific Ocean. The area of the island is 315 km². It is 42,5 km in length and 11-16,7 km wide.
Onekotan Island is unique because it has a water-filled caldera at both the northern and the southern ends of the island. The more prominent caldera at the southern end of the island has "an island within the island" and is known as the Tao-Rusyr Caldera. The highest elevation on Onekotan Island is Krenitsyn which rises to 1325 meters above sea level on the island that lies within the caldera. This volcano was named after Captain Pyotr Krenitsyn of the Imperial Russian Navy. More recent secondary volcanic cones have formed in both calderas; the northern caldera has almost been filled by the subsidiary cone, Nemo Peak.
There are chars in Lake Chyornoye, one of the two lakes on Onekotan, located at the northern end of the island.