(1769–1822), British statesman, born in county Down, Ireland, and educated at the University of Cambridge. In 1790 he entered the Irish parliament as a Whig, but he joined the Tory party when he entered the British House of Commons in 1795. A year later he was created Viscount Castlereagh, a courtesy title. As chief secretary for Ireland from 1799, he energetically supported the attempt of the British Prime Minister William Pitt the Younger to bring about the political union of Ireland with Great Britain. Pitt's proposed legislation, known as the Act of Union, was carried in the Irish parliament in 1800, largely through Castlereagh's skill in bribing parliamentary members. Soon after the act became law (Jan. 1, 1801), Castlereagh resigned from office because of the opposition of King George III to the passing of a Catholic emancipation act, which Castlereagh had hoped would follow the Act of Union.

Castlereagh was a member of the House of Commons from 1801 until his death, serving as leader from 1812. As secretary of state for the war and colonial department during most of the period from 1805 to 1809, he helped plan British campaigns in the Napoleonic Wars.

From 1812, as foreign secretary in the Tory cabinet of Robert Banks Jenkinson (1770–1828), 2d earl of Liverpool, Castlereagh played a leading part in the coalition of nations against Napoleon, keeping it united during the critical campaigns of 1813–14. He represented Great Britain at the Congress of Vienna (1814–15), which redrew the map of Europe after the Napoleonic Wars. At the Congress of Aix-la-Chapelle (1818), he resisted Russian attempts to draw Britain into a European league to oppose revolution.

In 1822, suffering from depression and fearful that he was about to be exposed as a homosexual, Castlereagh committed suicide.