William I, Count of Nassau-Dillenburg (d. 1559)
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. He was the brother of count Henry III of Nassau-Breda and the father of William I of Orange.

His eldest son William the Silent inherited the principality of Orange in Southern France from his cousin René of Chalon, as well as the vast properties of the House of Nassau-Dillenburg in the Netherlands from his father, which Engelbert I of Nassau had received by marriage in 1403. 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.