William III (14 November 1650 – 8 March 1702) (Dutch: Willem III) is a sovereign Prince of Orange by birth.

From 1672 he governs as Stadtholder William III of Orange (Dutch: Willem III van Oranje) over Holland, Zeeland, Utrecht, Guelders, and Overijssel of the Dutch Republic.

From 1689 he reigns as William III over England and Ireland, and as William II over Scotland.

He is informally known in Northern Ireland and Scotland as "King Billy".

A member of the House of Orange-Nassau, William wins the English, Scottish, and Irish crowns following the Glorious Revolution, in which his uncle and father-in-law James II is deposed.

In the British Isles, William rules jointly with his wife, Mary II, until her death on 28 December 1694.

Popular histories usually refer to the joint reign as that of "William and Mary".

A Protestant, William participates in several wars against the powerful Catholic king of France, Louis XIV, in coalition with Protestant and Catholic powers in Europe.

Many Protestants herald him as a champion of their faith.

Largely because of that reputation, William is able to take the British crowns when many are fearful of a revival of Catholicism under James.

William's victory over James II at the Battle of the Boyne in 1690 is commemorated by the Orange Institution in Northern Ireland to this day.

His reign markd the beginning of the transition from the personal rule of the Stuarts to the more-Parliament-centered rule of the House of Hanover.