Robert Arthur Talbot Gascoyne-Cecil, 3rd Marquess of Salisbury (/ˈɡæskɔɪn ˈsɪsəl/ GASK-oyn SISS-əl; 3 February 1830 – 22 August 1903), known as Lord Salisbury, was a British statesman and Conservative politician who served as Prime Minister of the United Kingdom three times for a total of over thirteen years. He was also Foreign Secretary before and during most of his tenure. He avoided international alignments or alliances, maintaining the policy of "splendid isolation".

Lord Robert Cecil, later known as Lord Salisbury, was first elected to the House of Commons in 1854 and served as Secretary of State for India in Lord Derby's Conservative government 1866–1867. In 1874, under Disraeli, Salisbury returned as Secretary of State for India, and, in 1878, was appointed foreign secretary, and played a leading part in the Congress of Berlin. After Disraeli's death in 1881, Salisbury emerged as the Conservative leader in the House of Lords, with Sir Stafford Northcote leading the party in the Commons. He succeeded William Ewart Gladstone as prime minister in June 1885, and held the office until January 1886.

When Gladstone came out in favour of Home Rule for Ireland later that year, Salisbury opposed him and formed an alliance with the breakaway Liberal Unionists, winning the subsequent 1886 general election. His biggest achievement in this term was obtaining the majority of the new territory in Africa during the Scramble for Africa, avoiding a war or serious confrontation with the other powers. He remained as prime minister until Gladstone's Liberals formed a government with the support of the Irish nationalists at the 1892 general election. The Liberals, however, lost the 1895 general election, and Salisbury for the third and last time became prime minister. He led Britain to victory in a bitter, controversial war against the Boers, and led the Unionists to another electoral victory in 1900. He relinquished the premiership to his nephew Arthur Balfour in 1902 and died in 1903. He was the last prime minister to serve from the House of Lords throughout the entirety of their premiership.

Historians agree that Salisbury was a strong and effective leader in foreign affairs, with a wide grasp of the issues. Paul Smith characterises his personality as "deeply neurotic, depressive, agitated, introverted, fearful of change and loss of control, and self-effacing but capable of extraordinary competitiveness." A representative of the landed aristocracy, he held the reactionary credo, "Whatever happens will be for the worse, and therefore it is in our interest that as little should happen as possible." Searle says that instead of seeing his party's victory in 1886 as a harbinger of a new and more popular Conservatism, Salisbury longed to return to the stability of the past, when his party's main function was to restrain what he saw as demagogic liberalism and democratic excess. He is generally ranked in the upper tier of British prime ministers.

Robert Arthur Talbot Gascoyne-Cecil was born on 3 February 1830 at Hatfield House, the third son of the 2nd Marquess of Salisbury and Frances Mary, née Gascoyne. He was a patrilineal descendant of Lord Burghley and the 1st Earl of Salisbury, chief ministers of Elizabeth I. The family-owned vast rural estates in Hertfordshire and Dorset. This wealth increased sharply in 1821, when his father married his mother, Frances Mary Gascoyne, heiress of a wealthy merchant and Member of Parliament who had bought large estates in Essex and Lancashire.

Robert had a miserable childhood, with few friends, and filled his time with reading. He was bullied unmercifully at the schools he attended. In 1840, he went to Eton College, where he did well in French, German, Classics, and Theology, but left in 1845 because of intense bullying. His unhappy schooling shaped his pessimistic outlook on life and his negative views on democracy. He decided that most people were cowardly and cruel, and that the mob would run roughshod over sensitive individuals.

In December 1847, he went to Christ Church, Oxford, where he received an honorary fourth class in Mathematics, conferred by nobleman's privilege due to ill health. Whilst at Oxford, he found the Oxford movement or "Tractarianism" to be an intoxicating force, and had an intense religious experience that shaped his life. He was involved in the Oxford Union, serving as its secretary and treasurer. In 1853, he was elected a prize fellow of All Souls College, Oxford.

In April 1850, he joined Lincoln's Inn, but did not enjoy law. His doctor advised him to travel for his health, and so, from July 1851 to May 1853, Cecil travelled through Cape Colony in southern Africa, Australia, including Tasmania, and New Zealand. He disliked the Boers and wrote that free institutions and self-government could not be granted to the Cape Colony because the Boers outnumbered the British three-to-one, and "it will simply be delivering us over bound hand and foot into the power of the Dutch, who hate us as much as a conquered people can hate their conquerors". He found the Kaffirs "a fine set of men – whose language bears traces of a very high former civilisation", similar to Italian. They were "an intellectual race, with great firmness and fixedness of will" but "horribly immoral" because they lacked theism.