William IV, Grand Duke of Luxembourg

William IV (French: Guillaume Alexandre; 22 April 1852 – 25 February 1912) was Grand Duke of Luxembourg from 17 November 1905 until his death in 1912. He succeeded his father, Adolphe. Like his father, William did not participate in politics, despite being vested with considerable power by the Constitution.

William was a Protestant, the religion of the House of Nassau. He married Infanta Marie Anne of Portugal, believing that Luxembourg, a Catholic country, should be headed by a Catholic monarch, thus making his successors Catholic.

At the death of his uncle, Prince Nikolaus-Wilhelm in 1905, the only other legitimate heir to the House of Nassau-Weilburg was William's cousin, Georg Nikolaus, Count of Merenberg, the product of a morganatic marriage. In 1907, William declared the Counts of Merenberg non-dynastic, declaring his eldest daughter Marie-Adélaïde as heiress presumptive to the grand ducal throne. She became grand duchess in her own right upon her father's death in 1912, and upon her own abdication in 1919, was succeeded by his younger daughter Charlotte.

To date, William IV is the last monarch of Luxembourg to die on the throne.