69°30′S 65°00′W / 69.500°S 65.000°W / -69.500; -65.000 The Antarctic Peninsula, known as O'Higgins Land in Chile and Tierra de San Martin in Argentina, and originally as Graham Land in the United Kingdom and the Palmer Peninsula in the United States, is the northernmost part of mainland Antarctica.

The Antarctic Peninsula is part of the larger peninsula of West Antarctica, protruding 1,300 km (810 miles) from a line between Cape Adams (Weddell Sea) and a point on the mainland south of the Eklund Islands. Beneath the ice sheet that covers it, the Antarctic Peninsula consists of a string of bedrock islands; these are separated by deep channels whose bottoms lie at depths considerably below current sea level. They are joined by a grounded ice sheet. Tierra del Fuego, the southernmost tip of South America, is about 1,000 km (620 miles) away across the Drake Passage.

The Antarctic Peninsula is 522,000 square kilometres (202,000 sq mi) in area and 80% ice-covered.

The marine ecosystem around the western continental shelf of the Antarctic Peninsula (WAP) has been subjected to rapid climate change. Over the past 50 years, the warm, moist maritime climate of the northern WAP has shifted south. This climatic change increasingly displaces the once dominant cold, dry continental Antarctic climate. This regional warming has caused multi-level responses in the marine ecosystem such as increased heat transport, decreased sea ice extent and duration, local declines in ice-dependent Adélie penguins, increase in ice-tolerant gentoo and chinstrap penguins, accelerated greening due to the spread of moss, alterations in phytoplankton and zooplankton community composition as well as changes in krill recruitment, abundance and availability to predators.

The Antarctic Peninsula is currently dotted with numerous research stations, and nations have made multiple claims of sovereignty. The peninsula is part of disputed and overlapping claims by Argentina, Chile, and the United Kingdom. None of these claims have international recognition and, under the Antarctic Treaty System, the respective countries do not attempt to enforce their claims. The British claim, however, is recognised by Australia, France, New Zealand, and Norway. Argentina has the most bases and personnel stationed on the peninsula.

The most likely first sighting of the Antarctic Peninsula, and therefore also of any part of the Antarctic mainland, was on 27 January 1820 by an expedition of the Imperial Russian Navy led by Fabian Gottlieb von Bellingshausen. But the party did not recognize as the mainland what they thought was an icefield covered by small hillocks.

Three days later, on 30 January 1820, Edward Bransfield and William Smith, with a British expedition, were the first to chart part of the Antarctic Peninsula. This area was later to be called Trinity Peninsula and is the extreme northeast portion of the peninsula. The next confirmed sighting was in 1832 by John Biscoe, a British explorer, who named the northern part of the Antarctic Peninsula as Graham Land.

The first European to land on the continent is also disputed. A 19th-century seal hunter, John Davis, was almost certainly the first. But, sealers were secretive about their movements and their logbooks were deliberately unreliable, to protect any new sealing grounds from competition.