King George Island (Argentina: Isla 25 de Mayo, Chile: Isla Rey Jorge, Russian historical name - Vaterlo (Waterloo)) is the largest of the South Shetland Islands, situated at , 120 kilometers off the coast of Antarctica in the Southern Ocean. The Island was named after King George III. It is not to be confused with King George Island, a small uninhabited island off South Eastern Tasmania, nor with George Island in the Falkland Islands.

The island was first claimed for Britain on 16 October 1819, formally annexed by Britain and claimed as part of the Falkland Islands Dependencies in 1908, and now as part of the separate British Antarctic Territory. The Island was claimed by Chile in 1940, now as part of the Chilean Antarctic Territory. It was claimed also by Argentina in 1943, now as part of Argentine Antarctica, called by the Argentines Isla Veinticinco de Mayo in honour of their National day. The USA and Russia do not recognize these claims, and have formally reserved their right to claim Antarctic territories.

The island was discovered by the British explorer William Smith in 1819. It is approximately 95 kilometres long and 25 kilometres wide with a land area of 1150 square kilometres. Over 90% of the island's surface is permanently glaciated. In 1821, 11 men of the sailing vessel Lord Melville survived the winter on the island, the first men to do so in Antarctica.

The coastal areas of the island are home to a comparatively diverse selection of vegetation and animal life, including Elephant Seals, Weddell and Leopard seals, and Chinstrap and Gentoo penguins.

Human habitation of King George Island is limited to research stations belonging to Argentina, Brazil, Chile, China, South Korea, Peru, Poland, Russia, and Uruguay. Most of these stations are permanently manned, carrying out research into areas as diverse as biology, ecology, geology, and palaeontology.

In 2004, a Russian Orthodox church, Trinity Church, was opened on the island near Russia's Bellingshausen Station. The church, one of the southernmost in the world and one of the few permanent structures in Antarctica, is permanently manned by a priest.

A small amount of specialised tourist activity also takes place during summer, including an annual marathon, known as the Antarctic marathon.

The Fildes Peninsula 7 km long (4.5 mi), forms the SW extremity of the island. It was named from association with nearby Fildes Strait by the UK-APC in 1960.