The Nimrod Expedition of 1907–1909, otherwise known as the British Antarctic Expedition, was the first of three successful expeditions to the Antarctic led by Ernest Shackleton and his second expedition to the Antarctic. Its main target, among a range of geographical and scientific objectives, was to be first to the South Pole. This was not attained, but the expedition's southern march reached a Farthest South latitude of 88° 23' S, just 97.5 nautical miles (180.6 km; 112.2 mi) from the pole. This was by far the longest southern polar journey to that date and a record convergence on either Pole. A separate group led by Welsh Australian geology professor Edgeworth David reached the estimated location of the South Magnetic Pole, and the expedition also achieved the first ascent of Mount Erebus, Antarctica's second highest volcano.

The expedition lacked governmental or institutional support, and relied on private loans and individual contributions. It was beset by financial problems and its preparations were hurried. Its ship, Nimrod, was less than half of the size of Robert Falcon Scott's 1901–1904 expedition ship Discovery, and Shackleton's crew lacked relevant experience. Controversy arose from Shackleton's decision to base the expedition in McMurdo Sound, close to Scott's old headquarters, in contravention of a promise to Scott that he would not do so. Nevertheless, although the expedition's profile was initially much lower than that of Scott's six years earlier, its achievements attracted widespread interest and made Shackleton a national hero. The scientific team, which included the future Australasian Antarctic Expedition leader Douglas Mawson, carried out extensive geological, zoological and meteorological work. Shackleton's transport arrangements, based on Manchurian ponies, motor traction, and sled dogs, were innovations which, despite limited success, were later copied by Scott for his ill-fated Terra Nova Expedition.

On his return, Shackleton overcame the Royal Geographical Society's (RGS) initial scepticism about his achievements and received many public honours, including a knighthood from King Edward VII. He made little financial gain from the expedition and eventually depended on a government grant to cover its liabilities. Within three years his southernmost record had been surpassed, as first Amundsen and then Scott reached the South Pole. In his own moment of triumph, Amundsen nevertheless observed: "Sir Ernest Shackleton's name will always be written in the annals of Antarctic exploration in letters of fire".