Onekotan (Russian: Онекотан; Japanese 温禰古丹島; Onekotan-tō, occasionally Onnekotan-tō, Ainu: オネコタン　or　オネコタㇴ) is an uninhabited volcanic island located near the northern end of the Kuril Islands chain in the Sea of Okhotsk in the northwest Pacific Ocean. Its name is derived from the Ainu language for "large village”. It is the second largest island, after Paramushir, in the northern subgroup of the Kurils. It is administratively included in the Severo-Kurilsky District of Sakhalin oblast, Russia.

Onekotan is roughly rectangular, with a length of 42.5 kilometers (26.4 mi), and a width ranging from 11 to 17 kilometers (6.8 to 10.6 mi). It has an area of 425 square kilometers (164 sq mi)

The island consists of two stratovolcanos connected by a relatively flat isthmus.

The contemporary structure of the caldera basin of Koltsevoe Lake was examined by Kozlov et al. through sonar survey utilizing a digital sonar and satellite positioning. The survey yielded 27 sonar profiles and a compiled bathymetric map. Presently, the lake holds the fourth position among Russia's deepest lakes, standing as the deepest in the Russian Far East and the largest in the Sakhalin region. Its significant water volume underscores its importance as a vital hydrological asset in the Far Eastern macro-region.

There are seven rivers on the island more than five kilometers long. The largest is Ozernaya River (8 kilometers (5.0 mi) long, which flows from the southern part of the caldera of the Nemo volcano to the Sea of Okhotsk. Almost the same length is the Kedrovka River, which flows from the central part of the island into the Pacific Ocean.

The climate of Onekotan is characterized by short, cool summers, high humidity, frequent fogs, and especially strong winds. However, the island's climate is generally considered to be mild oceanic with an average annual temperature of about 4.0 °C (39.2 °F). Even in record cold winters, temperatures below −15 °C (5 °F) were not observed.

Onekotan is separated from the neighboring islands by the Fourth Kuril Strait, Yevreinov Strait, and Krenitsyn Strait. The neighboring islands are Makanrushi, with a 1,170-meter (3,840 ft) volcanic peak, is 28 kilometers (17 mi) to the northwest of Onekotan; and Kharimkotan, with a 1,210-meter (3,970 ft) high volcano, is 15 kilometers (9.3 mi) to the southwest. Paramushir is located 53 kilometers (33 mi) to the northeast.

Onekotan still has an active volcano with the most recent eruption, in 1952, forming a small lava dome on the coast of Krenitsyn island within the lake of Tao-Rusyr Caldera. A bay on the eastern shore contains the remains of an abandoned settlement.