George III (George William Frederick; 4 June 1738 – 29 January 1820) was King of Great Britain and King of Ireland from 25 October 1760 until the union of these two countries on 1 January 1801, after which he was King of the United Kingdom of Great Britain and Ireland until his death. He was concurrently Duke and prince-elector of Brunswick-Lüneburg ("Hanover") in the Holy Roman Empire until his promotion to King of Hanover on 12 October 1814. He was the third British monarch of the House of Hanover, but unlike his two Hanoverian predecessors he was born in Britain, spoke English as his first language, and never visited Hanover.

His life and reign, which were longer than those of any previous British monarch, were marked by a series of military conflicts involving his kingdoms, much of the rest of Europe, and places farther afield in Africa, the Americas and Asia. Early in his reign, Great Britain defeated France in the Seven Years' War, becoming the dominant European power in North America and India. However, many of its American colonies were soon lost in the American War of Independence. Further wars against revolutionary and Napoleonic France from 1793 concluded in the defeat of Napoleon at the Battle of Waterloo in 1815.