| William | |
| Full Name: | Georg Wilhelm August Heinrich Belgicus |
| Succession: | Duke of Nassau |
| Reign: | 24 March 1816 – 20 August 1839 |
| Predecessor: | Frederick Augustus |
| Successor: | Adolf |
| Succession1: | Prince of Nassau-Weilburg |
| Reign1: | 9 January 1816 – 24 March 1816 |
| Predecessor1: | Frederick William |
| Successor1: | None. Incorporated into the Duchy of Nassau |
| Spouse: | |
| Issue: | Princess Auguste Therese, Duchess Peter Georgievich of Oldenburg Adolphe, Grand Duke of Luxembourg Prince Wilhelm Karl Prince Moritz Princess Marie Wilhelmine Prince Wilhelm Marie, Princess of Wied Helena, Princess of Waldeck and Pyrmont Prince Nikolaus Wilhelm Sophia, Queen of Sweden and Norway |
| House: | Nassau-Weilburg |
| Father: | Frederick William, Prince of Nassau-Weilburg |
| Mother: | Burgravine Louise Isabelle of Kirchberg |
| Birth Date: | 14 June 1792 |
| Birth Place: | Kirchheimbolanden, Principality of Nassau-Weilburg |
| Death Place: | Kissingen, Kingdom of Bavaria |
| Burial Place: | Schlosskirche, Weilburg |
| Religion: | Calvinism |
Wilhelm (Given names: Georg Wilhelm August Heinrich Belgicus; 14 June 1792, Kirchheimbolanden – 20/30 August 1839, Bad Kissingen) was joint sovereign Duke of Nassau, along with his father's cousin Frederick Augustus, reigning from 1816 until 1839. He was also sovereign Prince of Nassau-Weilburg from 1816 until its incorporation into the duchy of Nassau.
Frederick Augustus died on 24 March 1816 and Wilhelm inherited the Usingen territories and became sole sovereign of the Duchy of Nassau.
He is the father of Adolphe, Grand Duke of Luxembourg, and Queen Sophia of Sweden and Norway, consort of King Oscar II and a 3rd cousin of William III of the Netherlands, who left a surviving daughter to rule his main realm, but the crown of Luxembourg went through the male line, looking to 17 generations back, to pass to the Duke of Nassau and then his descendants.
Wilhelm was the eldest son of Frederick William, Duke of Nassau, and his wife, Burgravine Louise Isabelle of Kirchberg. With the Nassau troops, he was involved on the Seventh Coalition's side in the Battle of Waterloo against Napoleon.
On 9 January 1816, he succeeded his father, Duke Frederick William, as the Prince of Nassau-Weilburg and joint Duke of Nassau with his cousin, Frederick Augustus, of the Nassau-Usingen branch of his family. When his cousin and co-Duke died on 24 March 1816, Wilhelm inherited the Usingen territories and became sole ruler of the Duchy of Nassau. He kept the title of Duke of Nassau for the rest of his reign.
On 24 June 1813 in Weilburg, Wilhelm married his first wife, Princess Louise of Saxe-Altenburg (28 January 1794 Hildburghausen, Thüringen – 6 April 1825, Biebrich or Weilburg). She was a daughter of Frederick, Duke of Saxe-Altenburg (who, until 1826, had been the last Duke of Saxe-Hildburghausen) and his wife, Duchess Charlotte Georgine of Mecklenburg-Strelitz. They had eight children:
Wilhelm married, as his second wife, his first wife's niece, Princess Pauline of Württemberg (Stuttgart, 25 February 1810 – Wiesbaden, 7 July 1856) on 23 April 1829 in Stuttgart. Pauline was a daughter of Prince Paul of Württemberg and his wife Charlotte of Saxe-Hildburghausen.
Wilhelm and Pauline had four children: