William III (William Henry; Dutch: Willem Hendrik; 4 November 1650 8 March 1702), also widely known as William of Orange, was the sovereign Prince of Orange from birth, Stadtholder of Holland, Zeeland, Utrecht, Guelders, and Overijssel in the Dutch Republic from the 1670s, and King of England, Ireland, and Scotland from 1689 until his death in 1702. As King of Scotland, he is known as William II. He is sometimes informally known as "King Billy" in Ireland and Scotland. His victory at the Battle of the Boyne in 1690 is commemorated by Unionists, who display orange colours in his honour. He ruled Britain alongside his wife and cousin, Queen Mary II, and popular histories usually refer to their reign as that of "William and Mary".

William was the only child of William II, Prince of Orange, and Mary, Princess Royal, the daughter of King Charles I of England, Scotland, and Ireland. His father died a week before his birth, making William III the prince of Orange from birth. In 1677, he married his cousin Mary, the eldest daughter of his maternal uncle James, Duke of York, the younger brother and later successor of King Charles II.

A Protestant, William participated in several wars against the powerful Catholic French ruler Louis XIV in coalition with both Protestant and Catholic powers in Europe. Many Protestants heralded William as a champion of their faith. In 1685, his Catholic uncle and father-in-law, James, became king of England, Scotland, and Ireland. James's reign was unpopular with the Protestant majority in Britain, who feared a revival of Catholicism. Supported by a group of influential British political and religious leaders, William invaded England in what became known as the Glorious Revolution. In 1688, he landed at the south-western English port of Brixham; James was deposed shortly afterward.

William's reputation as a staunch Protestant enabled him and his wife to take power. During the early years of his reign, William was occupied abroad with the Nine Years' War (16881697), leaving Mary to govern Britain alone. She died in 1694. In 1696 the Jacobites, a faction loyal to the deposed James, plotted unsuccessfully to assassinate William and restore James to the throne. William's lack of children and the death in 1700 of his nephew Prince William, Duke of Gloucester, the son of his sister-in-law Anne, threatened the Protestant succession. The danger was averted by placing distant relatives, the Protestant Hanoverians, in line to the throne with the Act of Settlement 1701. Upon his death in 1702, the king was succeeded in Britain by Anne and as titular Prince of Orange by his cousin John William Friso, beginning the Second Stadtholderless period.

The Glorious Revolution of November 1688 (Irish: An Réabhlóid Ghlórmhar; Scottish Gaelic: Rèabhlaid Ghlòrmhor; Welsh: Chwyldro Gogoneddus), the invasion also known as the Glorieuze Overtocht or Glorious Crossing by the Dutch, was the deposition of James II and VII, king of England, Scotland and Ireland and replacement by his daughter Mary II and her husband, William III of Orange, stadtholder and de facto ruler of the Dutch Republic. A term first used by John Hampden in late 1689, historian Jeremy Black suggests it can be seen as both the last successful invasion of England and also an internal coup.Despite his Catholicism, James became king in February 1685 with widespread support as many feared his exclusion would lead to a repetition of the 1638–1651 Wars of the Three Kingdoms. Over the next three years, he alienated his supporters by suspending the Scottish and English Parliaments in 1685 and ruling by personal decree. Despite this, it was considered a short-term issue, as James was 52, and since his second marriage was childless after 11 years, the heir presumptive was his Protestant daughter Mary.

Two events in June 1688 turned dissent into a political crisis. The first was the birth of James Francis Edward on 10 June, displacing Mary as heir which created the prospect of a Catholic dynasty. The second was the prosecution of the Seven Bishops on 15 June; one in a series of perceived assaults on the Church of England, their acquittal on the 30th sparked anti-Catholic riots and destroyed James's political authority. The combination convinced a broad coalition of English politicians to issue an Invitation to William, inviting him to militarily intervene to protect the Protestant religion.

With Louis XIV of France preparing to attack the Dutch, William viewed this as an opportunity to secure English resources for the Nine Years' War, which began in September 1688. On 5 November, he landed in Brixham in Torbay with 14,000 men. As he advanced on London, most of the 30,000-strong Royal Army joined him. James went into exile on 23 December and in April 1689, Parliament made William and Mary joint monarchs of England and Ireland. A separate but similar Scottish settlement was made in June.

While the Revolution itself was quick and relatively bloodless, pro-Stuart revolts in Scotland and Ireland caused significant casualties. Although Jacobitism persisted into the late 18th century, the Revolution ended a century of political dispute by confirming the primacy of Parliament over the Crown, a principle established in the Bill of Rights 1689. The Toleration Act 1688 granted freedom of worship to nonconformist Protestants, but restrictions on Catholics contained in the 1678 and 1681 English and Scottish Test Acts remained in force until 1828; while religious prohibitions on the monarch's choice of spouse were removed in 2015, those applying to the monarch remain.