| William IV | |
| Succession: | Grand Duke of Luxembourg |
| Reign: | 17 November 1905 – 25 February 1912 |
| Predecessor: | Adolphe |
| Successor: | Marie-Adélaïde |
| Regent: | Grand Duchess Maria Anna |
| Regent1: | Paul Eyschen |
| Spouse: | Infanta Marie Anne of Portugal (m. 1893) |
| Issue: | Marie-Adélaïde, Grand Duchess of Luxembourg Charlotte, Grand Duchess of Luxembourg Hilda, Princess of Schwarzenberg Antonia, Crown Princess of Bavaria Elisabeth, Princess Ludwig Philipp of Thurn and Taxis Sophie, Princess Ernst Heinrich of Saxony |
| House: | Nassau-Weilburg |
| Father: | Adolphe, Grand Duke of Luxembourg |
| Mother: | Princess Adelheid-Marie of Anhalt-Dessau |
| Religion: | Protestantism |
| Birth Date: | 22 April 1852 |
| Birth Place: | Biebrich Palace, Wiesbaden, Duchy of Nassau |
| Death Place: | Berg Castle, Colmar-Berg, Luxembourg |
| Burial Place: | Schlosskirche ("Castle Church") in Weilburg since 1953 |
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 mostly stayed out of politics despite being vested with considerable power on paper by the Constitution.
William was a Protestant, the religion of the House of Nassau. He married Infanta Marie Anne of Portugal, believing that a Roman Catholic country ought to have a Roman Catholic monarch. Thus his heirs have been Catholic.
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). Charlotte's descendants reign until the present day.
To date, William is the last monarch of Luxembourg to die on the throne.
On 21 June 1893 in Fischhorn Castle, Zell am See, he married Infanta Marie Anne of Portugal, daughter of former King Miguel I of Portugal and Princess Adelaide of Löwenstein-Wertheim-Rosenberg. The couple had six daughters:
Although the duchy of Nassau was annexed by Prussia after the Austro-Prussian war of 1866, the title of Duke of Nassau was retained by William and his heirs.[1]
Denmark: Knight of the Elephant, 23 April 1876[7]
Netherlands: Grand Cross of the Netherlands Lion
Sweden-Norway: