Kunashir (Russian: Кунаши́р, romanized: Kunashír), known in Japan as Kunashiri (Japanese: 国後島, Hepburn: Kunashiri-tō), is the southernmost island of the Kuril Islands. The island has been under Russian administration since the end of World War II, when Soviet forces took possession of the Kurils. It is claimed by Japan as part of its "Northern Territories".

According to the Hokkaido Government, the Japanese name Kunashiri (国後島, Kunashiri-tō) comes from the Ainu kina-sir (キナシㇼ), which translates to "grass island".

Kunashir is separated by the Catherine Strait (Kunashiri Suido) from the island of Iturup, which is located 22 km northeast; Kunashir Strait (upper Nemuro Kaikyo) separates Kunashir from Shiretoko Peninsula (Hokkaido Island), located 25 km to the west; Izmena Strait (Notsuke Suido or lower Nemuro Kaikyo) separates Kunashir from Notsuke Peninsula (Hokkaido Island), located 16 km to the southwest; and the South Kuril Strait (Yuzhno-Kurilski Proliv, Minami Chishima Kaikyo) separates Kunashir from Shikotan and the Habomai Islands, 50 km to the east. Kunashir is visible from the nearby Japanese island of Hokkaido, from which it is separated by the Nemuro Strait.

Kunashir is formed by four volcanoes which were separate islands but have since become joined together by low-lying areas with lakes and hot springs. All four volcanoes are still active: Tyatya (1,819 m (5,968 ft)), Smirnov, Mendeleyeva (Rausu-yama), and Golovnin (Tomari-yama). The island is made up of volcanic and crystalline rocks.

Kunashir, the southernmost island of the Kuril Arc, is 123 km long (NE to SW), 7 to 30 km wide and has an area of 1,490 km². The island has several volcanic features, including active ones such as Tyatya (1,819 m), Ruruy (1,485 m), Mendeleev (886 m) and Golovnin (541 m). The terrain of Kunashir is rugged and mountainous, with young geomorphology, steep slopes and numerous waterfalls. Asymmetry in relief is notable, with the western (Okhotsk) shore being steep and elevated, in contrast to the eastern (Pacific) shore, which has a more level and flat topography.^{[citation needed]}

In contrast to the Japan Arc, the absolute timing of the evolution of the primary Kuril Arc system has received comparatively little attention, particularly with respect to modern geochronological techniques. A recent study (de Grave, 2015) aimed to fill this gap by presenting the initial zircon U/Pb ages of the volcanic basement and thermochronological ages for the rocks of Kunashir, thus establishing a definitive temporal framework for their emplacement and subsequent exhumation. The volcanic rocks of the Kuril Islands generally exhibited a two-level structural classification: (1) a lower level consisting mainly of moderately deformed Neogene rocks, and (2) an upper level comprising Pleistocene to recent volcanics. These rocks exhibited a wide range of compositions from basalt to rhyolite, with a predominance of (basaltic) andesites (Martynov et al.).

The geology of Kunashir could be elucidated from this perspective by delineating two structural levels. The lower level, termed the Lower Complex, was characterised by yellowish to yellow-grey tuffs, tuffaceous sandstones and breccias, mainly of felsic to medium composition. Numerous subvolcanic stocks and plugs of andesite, dacite and rhyolite intruded these formations, with deeper intrusive rocks of granodiorite porphyry composition and texture (Vergunov; Vergunov and Vlasov; Sergeev; Piskunov and Rybin). The upper level, termed the Upper Complex, was characterised by basaltic and basaltic andesite flows, small sub-volcanic intrusive bodies and modern andesitic stratovolcanoes.

Previous studies (Davydov et al.; Vitukhin et al.) suggested a late Miocene to Pliocene age for the Lower Complex, and accordingly older rocks were either absent or not clearly identified on the island. The Lower Complex has been further subdivided into the Rybakov and Kamuy formations (e.g. Zhelubovsky and Pryaluhina; Bevz), although Martynov et al. used a different classification and terminology (see below). The Rybakov Formation, located in the most uplifted and deeply exhumed blocks, especially in the northern part of the island, was mainly an andesitic volcanic complex. This formation correlates with what Martynov et al. termed the Miocene Greentuff Formation. The Kamuy Formation, which forms the backbone of the island, consisted mainly of flysch-like volcanic-sedimentary deposits rich in felsic pumice of (rhyo)dacitic composition, with a thickness exceeding 1100 m (Martynov et al.). The Upper Complex, represented by the Fregat Formation, overlies the Rybakov and Kamuy Formations (Lower Complex). A distinct structural and erosional unconformity separated these complexes. The Fregat Formation comprised subaerial (basaltic) andesitic lava flows interbedded with tuffs, hyaloclastites and basaltic breccias (Syvorotkin and Rusinova). This suggests that the Fregat volcanic rocks are predominantly of sub-aerial origin, occasionally deposited at shallow depths below sea level. The Fregat volcanic deposits formed a distinctive volcanic plateau, formed near sea level and affected by subsequent tectonic movements, resulting in the fragmented table-top mountainous topography of Kunashir (Syvorotkin and Rusinova, 1989). The age of the Fregat Formation was determined to be late Pliocene to early Pleistocene based on diatoms (Neodenticula kamtschatica – N. Koizumii) and palynology (Dunichev, 1969), in agreement with K-Ar data from the Fregat basalts.