Bouillon, town (1991 pop. 5,468), Luxembourg prov., SE Belgium, in the Ardennes on the Semois River, near the French border. It is a small manufacturing and tourist center. Its old castle belonged to Godfrey of Bouillon, one of the leaders of the First Crusade, who pledged (1095) the town and the surrounding duchy to the bishop of Liège to raise funds for the Crusade. Bouillon was nominally under the suzerainty of the prince-bishops of Liège until it passed (15th cent.) to William de la Marck, the "Boar of the Ardennes," whose descendants assumed the titles duke of Bouillon and prince of Sedan. The duchy was taken (1676) by Louis XIV of France and given to the La Tour d'Auvergne family. It was under direct French rule from 1794 to 1815, when it passed to the Netherlands. It became part of Belgium in 1830.