Robert Stewart, 2nd Marquess of Londonderry, KG, GCH, PC (18 June 1769 in Dublin – 12 August 1822 at Loring Hall, Kent), generally known by his courtesy title of Viscount Castlereagh, which he held until 1821, was an Anglo-Irish politician who represented the United Kingdom at the Congress of Vienna. He was also intimately involved in securing the passage of the controversial Irish Act of Union.