William I of Poitiers-Valentinois

William I of Poitiers (Guillaume de Poitiers) was the son of Count Aymar I. Around 1155 he married Beatrice, daughter of Guigues IV, count of Albon, allying his house with the Dauphins of Vienne, and he is first attested as count of Valentinois in 1158. In 1163 he did homage to the bishop of Die for Suze and Gigors in the Diois.

At Arles on 30 June 1178 he attended the coronation of Frederick Barbarossa, winning imperial privileges including the Rhône toll between Valence and Montélimar. He died after 4 May 1187 and was succeeded by his son Aymar II.