William I, Marquis of Namur, the Rich, (1324 – 1 October 1391) was Count of Namur from 1337 until his death.
He was the fifth son of John I and Mary of Artois.
Because his four elder brothers all died childless between 1333 and 1337, he still became Marquis of Namur.
William participated in the Hundred Years' War on the side of the English.
Despite this, Namur itself remained at peace, except for a revolt of the weavers in 1351.
Thanks to the fortune, first of his mother and then of his wife, William was able to buy several territories which enlarged the Marquisate. In 1362 he obtained from Charles IV, Holy Roman Emperor that Namur depended directly from the Holy Roman Empire and ceased to be a vassal of the County of Hainaut. He also developed the mining and forging industries, as well as commerce.
He first married on February 13 1348 with Jeanne of Hainault, daughter of John of Beaumont and Margaret of Soissons. Jeanne died 2 years later without children.
William remarried in March 1352 with Catherine of Savoy, daughter of Louis II of Savoy, baron of Vaud, and Isabella of Châlon. They had 3 children :
William also had several illegitimate children.