), German: Wilhelm Moritz Fürst von Nassau-Siegen, official titles: Fürst zu Nassau, Graf zu Katzenelnbogen, Vianden, Diez, Limburg und Bronkhorst, Herr zu Beilstein, Stirum, Wisch, Borculo, Lichtenvoorde und Wildenborch, Erbbannerherr des Herzogtums Geldern und der Grafschaft Zutphen, was a count from the House of Nassau-Siegen, a cadet branch of the Ottonian Line of the House of Nassau.

[1] William Maurice was born at Wisch Castle [nl] in Terborg on 18/28 January 1649[2][note 1] as the eldest son of Count Henry of Nassau-Siegen and Countess Mary Magdalene of Limburg-Stirum.

On 29 April 1663, William Maurice became a hopman of a company of Swiss soldiers in the Dutch States Army.

[2] During his reign, William Maurice had the Nassauischer Hof [de], the Residenz of the Protestant princes of Nassau-Siegen in the city of Siegen, extended.

[5] In 1690, he had the members of his dynasty, who had been buried in the St. Nicholas Church [de] in Siegen, transferred to the Fürstengruft [nl] there.

William Maurice died in the Nassauischer Hof in Siegen[16] on 23 January 1691Jul.,[2][note 3] and was buried in the Fürstengruft there on 12 March.