The King of Hanover was the official title of the head of state and hereditary ruler of the Kingdom of Hanover, beginning with the proclamation of King George III of the United Kingdom, as "King of Hanover" during the Congress of Vienna, on 12 October 1814 at Vienna, and ending with the kingdom's annexation by Prussia on 20 September 1866.

The Kingdom of Hanover was established in October 1814 by the Congress of Vienna, with the restoration of George III to his Hanoverian territories after the Napoleonic era. It succeeded the former Electorate of Hanover, and joined 38 other sovereign states in the German Confederation in June 1815. The kingdom was ruled by the House of Hanover, a cadet branch of the House of Welf, in personal union with Great Britain between 1714 and 1837. Since its monarch resided in London, a viceroy, usually a younger member of the British royal family, handled the administration of the Kingdom of Hanover.