William I, Count of Nassau-Dillenburg (nicknamed William the Rich, Dutch: Willem de Rijke; 10 April 1487 – 6 October 1559) was a count of Nassau-Dillenburg from the House of Nassau.
His nickname the Rich refers to him having many children.
However, he owned a number of counties: Nassau-Dillenburg, Nassau-Siegen, Nassau-Dietz and Vianden.
William was born in Dillenburg as the younger son of Count John V of Nassau-Dillenburg and Landgravine Elisabeth, daughter of Landgrave Henry III of Hesse-Marburg and Anna of Katzenelnbogen.
The early House of Orange-Nassau descends from William I., the Silent, while the later House of Orange-Nassau (and the Dutch royal family) descends in the male line from his younger brother John, Count of Nassau-Dillenburg, and from the latter's fifth son, Count Ernst Casimir of Nassau-Dietz, however in the female line also from William of Orange.