The County of Nassau was a German state within the Holy Roman Empire and later part of the German Confederation. Its ruling dynasty, the male line of which is now extinct, was the House of Nassau.

William I of Nassau-Siegen, nicknamed the Elder or the Rich, was Count of Nassau-Siegen and half of Diez from 1516 to 1559. He was a descendant of the Ottonian Line of the House of Nassau.