William IV (William Henry; 21 August 1765 – 20 June 1837) was 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, was the last king and penultimate monarch of the House of Hanover.

During his youth, he served in the Royal Navy; he was, both during his reign and afterwards, nicknamed the Sailor King. His reign was one of several reforms: the poor law updated, municipal government democratised, child labour restricted and slavery abolished throughout the British Empire. The most important reform legislation of William IV's reign was the Reform Act 1832, which refashioned the British electoral system. William did not engage in politics as much as his brother or his father, though he did prove to be the most recent monarch to appoint a Prime Minister contrary to the will of Parliament (in 1834). He gave a liberal constitution to the Kingdom of Hanover, his other kingdom.

William had two legitimate children who survived long enough to be christened and therefore named, Princess Charlotte and Princess Elizabeth; both died as infants. He had ten children out of wedlock with Dorothea Jordan, George FitzClarence, 1st Earl of Munster among them. Though nine of his illegitimate children survived him, neither of his legitimate children did and William was succeeded by his niece, Queen Victoria.