William IV (William Henry; 21 August 1765 – 20 June 1837) is King of the United Kingdom of Great Britain and Ireland and of Hanover from 26 June 1830 until his death.

William, the third son of George III and younger brother and successor to George IV, is the last king and penultimate monarch of Britain's House of Hanover.

He servesin the Royal Navy in his youth and is, both during his reign and afterwards, nicknamed the "Sailor King".

He serves in North America and the Caribbean, but sees little actual fighting.

Since his two older brothers had died without leaving legitimate issue, he inherits the throne when he is 64 years old.

His reign sees several reforms: the poor law is updated, child labor restricted, slavery abolished in nearly all the British Empire, and the Reform Act 1832 refashions the British electoral system.

Though William does not engage in politics as much as his brother or his father, he is the last monarch to appoint a Prime Minister contrary to the will of Parliament.

Through his brother, the Viceroy of Hanover, he grants that kingdom a short-lived liberal constitution.

At his death, William has no surviving legitimate children, though he is survived by eight of the ten illegitimate children he had had by the actress Dorothea Jordan, with whom he had cohabited for 20 years.

William is succeeded in the United Kingdom by his niece, Victoria, and in Hanover by his brother, Ernest Augustus I.