the name of a countship in France, the chief town of which was Valence (Drome). From the 12th to the 15th century Valentinois belonged to a family of Poitiers, which must not be confused with that of the counts of Poitiers. To the detriment of his kinsmen, the lords of St Vallier, Count Louis II. (d. 1419) bequeathed his counties of Valentinois and Diois to the Dauphin Charles, afterwards King Charles VII.; and in 1498 Louis XII. erected the countship of Valentinois into a duchy, and gave it to Caesar Borgia, son of Pope Alexander VI. A few years later Borgia was deprived of the duchy, which, in 1548, was given by Henry II. to his mistress, Diane de Poitiers, a descendant of the counts of Valentinois. Having again reverted to the Crown, the duchy was given by Louis XIII. to Honore Grimaldi, prince of Monaco, whose descendants retained it until the French Revolution. The new duchy of Valentinois, however, did not consist of the lands attached to the former one, but was made up of several scattered lordships in Dauphine. The title of duke of Valentinois is still borne by the prince of Monaco.
See J. Chevalier, Memoires pour servir d l'histoire des comtes de Valentinois et de Diois (Paris, 1897-1906).