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.

At the death of his uncle, Prince Nikolaus-Wilhelm in 1905, the only other legitimate male in the House of Nassau-Weilburg was William's cousin, Georg Nikolaus, Count of Merenberg, the product of a morganatic marriage.

So in 1907, William declared the Counts of Merenberg non-dynastic, naming his own eldest daughter Marie-Adélaïde (1894–1924) as heiress presumptive to the grand ducal throne.

She became Luxembourg's first reigning grand duchess upon her father's death in 1912, and upon her own abdication in 1919, was succeeded by her younger sister Charlotte (1896–1985).

To date, William is the last monarch of Luxembourg to die on the throne, and the last to bear a regnal number.