Sir Clements Robert Markham KCB FRS FRSGS (20 July 1830 – 30 January 1916) was an English geographer, explorer and writer. He was secretary of the Royal Geographical Society (RGS) between 1863 and 1888, and later served as the Society's president for a further 12 years. In the latter capacity he was mainly responsible for organising the British National Antarctic Expedition of 1901–1904, and for launching the polar career of Robert Falcon Scott.

Markham began his career as a Royal Navy cadet and midshipman, during which time he went to the Arctic with HMS Assistance in one of the many searches for Franklin's lost expedition. Later, Markham served as a geographer to the India Office, and was responsible for the collection of cinchona plants from their native Peruvian forests, and their transplantation in India. By this means, the Indian government acquired a home source from which quinine could be extracted. Markham also served as geographer to Sir Robert Napier's Abyssinian expeditionary force, and was present in 1868, at the fall of Magdala.

The main achievement of Markham's RGS presidency was the revival at the end of the 19th century of British interest in Antarctic exploration, after a 50-year interval. He had strong and determined ideas about how the National Antarctic Expedition should be organised, and fought hard to ensure that it was run primarily as a naval enterprise, under Scott's command. To do this he overcame hostility and opposition from much of the scientific community. In the years following the expedition he continued to champion Scott's career, to the extent of disregarding or disparaging the achievements of other contemporary explorers.

All his life Markham was a constant traveller and a prolific writer, his works including histories, travel accounts and biographies. He authored many papers and reports for the RGS, and did much editing and translation work for the Hakluyt Society, of which he also became president in 1890. He received public and academic honours, and was recognised as a major influence on the discipline of geography, although it was acknowledged that much of his work was based on enthusiasm rather than scholarship. Among the geographical features bearing his name is Antarctica's Mount Markham, named after him by Scott in 1902.

Markham was born on 20 July 1830, at Stillingfleet, now in North Yorkshire, the second son of the Reverend David Frederick Markham, then vicar of Stillingfleet. The family were descendants of William Markham, former Archbishop of York and royal tutor; this Court connection led to David Markham's appointment, in 1827, as an honorary canon of Windsor. Markham's mother Catherine, née Milner, was the daughter of Sir William Milner, 4th Baronet, of Nun Appleton Hall, Yorkshire.

In 1838, David Markham was appointed rector of Great Horkesley, near Colchester, Essex. A year later, Clements began his schooling, first at Cheam School, and later at Westminster School. Reportedly an apt pupil, he showed particular interest in geology and astronomy, and from an early age he wrote prolifically, an activity which filled much of his spare time. At Westminster, which he found "a wonderful and delightful place", he developed a particular interest in boating, often acting as coxswain in races on the River Thames.

In May 1844, Markham was introduced by his aunt, the Countess of Mansfield, to Rear Admiral Sir George Seymour, a Lord of the Admiralty. The boy made a favourable impression on the admiral, and the meeting led to the offer of a cadetship in the Royal Navy. Accordingly, on 28 June 1844, Markham travelled to Portsmouth to join Seymour's flagship, HMS Collingwood. Collingwood was being fitted out for an extended voyage to the Pacific Ocean where Seymour was to assume command of the Pacific station. This tour of duty lasted for almost four years. The ship reached the Chilean port of Valparaíso, the headquarters of the Pacific station on 15 December 1844, after a cruise that incorporated visits to Rio de Janeiro and the Falkland Islands, and a stormy passage in the Southern Ocean. Markham's social connections assured him of a relatively comfortable time; he was frequently invited to dine with the admiral, whose wife and daughters were on board.

After a few weeks' respite in Valparaiso, Collingwood sailed again, this time for Callao, the main port on the Peruvian coast, giving Markham his first experience of a country that would figure prominently in his later career. During the next two years Collingwood cruised in the Pacific, visiting the Sandwich Islands, Mexico, and Tahiti, where Markham attempted to assist the nationalist rebels against their French governor. On 25 June 1846, Markham passed the examination for midshipman, being placed third in a group of ten. The long periods spent in Chilean and Peruvian ports had also enabled him to learn Spanish.