Cook Island is the central and largest island of the Southern Thule island group, part of the South Sandwich Islands in the far south Atlantic Ocean. Southern Thule was discovered by a British expedition under Captain James Cook in 1775. Cook Island was named for Cook by a Russian expedition under Fabian Gottlieb von Bellingshausen, which explored the South Sandwich islands in 1819–1820.

Southern Thule is a group of the three southernmost islands in the South Sandwich Islands in the southern Atlantic Ocean: Bellingshausen, Cook, and Thule (Morrell). It is a largely submerged volcano of which only the three islands emerge above sea level. Between Cook and Thule, and south of Bellingshausen, lie two submerged calderas; a third caldera is located on Thule. Cook Island is inactive and largely glaciated, while Bellingshausen and Thule feature active craters with fumarolic activity, and there is evidence of eruptions in the 20th century.