Spencer Compton Cavendish, 8th Duke of Devonshire (1833 - 1908) was a British politician, previously known (1858-1891) as Marquess of Hartington.

Cavendish was educated at Trinity College, Cambridge and entered Parliament in 1857. Between 1863 and 1874 Hartington held various Government posts, including lord of the Admiralty, under-secretary for war, postmaster-general, and chief secretary for Ireland. In 1875 he became Leader of the Liberal opposition, and in 1880, after the fall of Benjamin Disraeli's government, he was invited to form a government, but chose instead to serve under William Ewart Gladstone as Secretary of State for India (1880-1882) and Secretary of State for War (1882-1885). In 1886 he broke with Gladstone over Irish Home Rule, and became the leader of the Liberal Unionists. Queen Victoria asked him twice more in 1886-7 to form a government, but again he declined, giving support from the back benches to the Conservative government of Lord Salisbury. Devonshire eventually joined Salisbury's government in 1895 as Lord President of the Council. He resigned from the government in 1903, and from the Liberal Unionist Association the following spring, in protest at Joseph Chamberlain's Tariff Reform scheme.