William III (14 November 1650 - 8 March 1702) was the Prince of Orange from his birth, Stadtholder of the main provinces of the Dutch Republic from 28 June 1672, King of England and King of Ireland from 13 February 1689, and King of Scots (under the name William II) from 11 April 1689, in each case until his death.

Born a member of the House of Orange-Nassau, William III won the English, Scottish and Irish Crowns following the Glorious Revolution, during which his uncle and father-in-law, James II, was deposed. In England, Scotland and Ireland, William ruled jointly with his wife, Mary II, until her death on 28 December 1694. He reigned as 'William II' in Scotland, but 'William III' in all his other realms. Often he is referred to as William of Orange, a name he shared with many other historical figures. In Northern Ireland and Scotland, he is often informally known as King Billy.

William III was appointed to the Dutch post of Stadtholder on 28 June 1672 (Old Style), and remained in office until he died. In that context, he is sometimes referred to as 'William Henry, Prince of Orange', as a translation of his Dutch title, Willem Hendrik, Prins van Oranje. A Protestant, William participated in many wars against the powerful Catholic King Louis XIV of France.

Many Protestants heralded him as a champion of their faith. It was largely due to that reputation that he was able to take the crowns of England and Scotland, where many, especially the English aristocracy, were intensely fearful of a revival of Catholicism and the papacy. Undoubtedly, his army and sizable naval fleet also played a role.

His reign marked the beginning of the transition from the crown's personal control of government of the Stuarts to the more Parliament-centered rule of the House of Hanover.