Maria of Austria, Duchess of Jülich-Cleves-Berg

William of Jülich-Cleves-Berge (William I of Cleves, William V of Jülich-Berg) (German: Wilhelm der Reiche; 28 July 1516 – 5 January 1592) was a Duke of Jülich-Cleves-Berg (1539–1592). William was born in and died in Düsseldorf. He was the only son of John III, Duke of Jülich-Cleves-Berg, and Maria, Duchess of Jülich-Berg. William took over rule of his father's estates (the Duchy of Cleves and the County of Mark) upon his death in 1539. Despite his mother having lived until 1543, William also became the Duke of Berg and Jülich and the Count of Ravenstein.

From 1538 to 1543, William held the neighbouring Duchy of Guelders, as successor of his distant relatives, the Egmond dukes. Emperor Charles V claimed this duchy for himself as the dukes had sold their right of heritage, and William tried to hold on to it. He made a treaty with the King of France and married Jeanne d'Albret, and with this backup dared to challenge the Emperor. All too soon he learned that the French did not lift a finger to help him, and he was overwhelmed and had to surrender. In accordance with the Treaty of Venlo (1543) that was the result of this war, Guelders and the County of Zutphen were transferred to Charles V, Holy Roman Emperor, combining them with the Habsburg Netherlands.

William then tried to strengthen his inherited territories and launched an impressive development project for the most important cities. The three duchies all got new main fortresses as major strongpoints, for the older medieval fortifications had proved to be no match against the Imperial artillery. The cities of Jülich, Düsseldorf and Orsoy became fortresses for the duchies of Jülich, Berg and Cleves respectively, and Jülich and Düsseldorf were turned into impressive residences. For this task, the renowned Italian architect Alessandro Pasqualini from Bologna was hired, who had already made some impressive display of his craft in the Netherlands. He made the plans for the fortifications and palaces, of which some traces still remain, especially at Jülich where the citadel (built 1548-1580) is a major landmark, with parts of the Renaissance palace still standing.

William's sister Anne of Cleves was, for six months, the fourth wife of King Henry VIII of England. Through his daughter Marie Eleonore, he is ancestor of Marie Louise of Hesse-Kassel, wife of John William Friso, Prince of Orange therefore ancestor of all the current European monarchs.