William, Count of Nassau-Siegen, German: Wilhelm Graf von Nassau-Siegen, official titles: Graf zu Nassau, Katzenelnbogen, Vianden und Diez, Herr zu Beilstein, was Count of Nassau-Siegen, a part of the County of Nassau from 1624 to 1642. A member of the House of Nassau-Siegen, a cadet branch of the Ottonian Line of the House of Nassau, he was a professional soldier who served in the armies of the Hanseatic League and the Republic of Venice, then with the Dutch States Army during the Eighty Years War. Promoted field marshal in 1633, he was successively governor of Emmerich, Heusden and Sluis.

Nassau-Siegen was a principality within the Holy Roman Empire that existed between 1303 and 1328, and again from 1606 to 1743. From 1626 to 1734, it was subdivided into Catholic and Protestant parts. Its capital was the city of Siegen, founded in 1224 and initially a condominium jointly owned by the archbishopric of Cologne and Nassau. It was located some 50 km east of Cologne, and it contained the modern localities of Freudenberg, Hilchenbach, Kreuztal, Siegen, and Wilnsdorf.