Chateaudun, a town of France, in the department of Eure-et-Loir, on the left bank of the Loir, 25 m. S. of Chartres and 30 m. N. W. of Orleans; pop. in 1866, 6,781. It is prettily situated on a commanding elevation, and contains a line square and promenade, and a number of handsome public buildings. The principal churches are the Madeleine and St.Valerien, built in the 12th century. The most conspicuous edifice is the ancient castle of the counts Dunois, built on a rock and surmounted by a high tower, with a famous staircase and other curious appendages. The trade is mainly in the local agricultural and industrial products, including nails, cordage, leather, woollen and silk goods, hosiery, and hats. The viscounts of Chateaudun ruled here from the 11th to the loth century, when the last of them dying childless, Duke Charles of Orleans became the local ruler; he relinquished his supremacy in favor of his natural brother John, the bastard of Orleans, famous for his exploits in the wars with England. The town was destroyed by fire in 1723, and rebuilt on an improved plan.

During the war of 1870-'71 the French were defeated here by Gen. Willich, who soon after took the town by assault, Oct. 18, 1870.