William I of Champlitte () (1160s-1209) was a French knight who joined the Fourth Crusade and became the first prince of Achaea (1205–1209). Early years and the Fourth Crusade William was the second son of Odo or Eudes I of Champlitte, viscount of Dijon (son of Hugh, Count of Champagne) and his wife, Sybille. He married first Alais, the lady of Meursault. With the consent of his wife, he donated property to the Cistercian abbey of Auberive for the soul of his younger brother, Hugh in 1196. He later married Elisabeth of Mount-Saint-Jean, but they divorced in 1199. William and his brother, Odo II of Champlitte joined the Fourth Crusade in September 1200 at Cîteaux. William was one of the crusader leaders who signed the letter written in April 1203 by Counts Baldwin IX of Flanders, Louis I of Blois and Chartres and Hugh IV of Saint Pol to Pope Innocent III who had excommunicated the whole expedition after the occupation of Zara (now Zadar, Croatia). They begged the pope not to chastise Marques Boniface I of Montferrat, the leader of the crusade who had, in order to preserve the integrity of the expedition, withheld the publication of the papal bull of anathema. The crusaders took Constantinople on April 13, 1204. They gave the imperial throne to Baldwin IX of Flanders who was ceremoniously crowned on May 16, 1204, but William of Champlitte joined to Boniface of Montferrat who became king of Thessalonica under the new emperor. According to the Partitio Romaniae, a treaty concluded by all the leaders of the Fourth Crusade, the Republic of Venice received title to occupy, among other territories, the whole Peloponnese (in modern Greece). Foundation of the Principality of Achaea Early in 1205 Geoffrey of Villehardouin, one of William of Champlitte's old friends arrived to the camp of Boniface I of Thessalonica at Nauplia (now Nafplion, Greece). He had earlier occupied some parts of Messenia and now persuaded the king that even though the northeastern part of the Peloponnese was offering resistance the rest of the peninsula could easily be conquered. Geoffrey also offered to share the territory with William. Boniface I thereupon appointed William to hold the Peloponnese as a fief from him. Geoffrey of Villehardouin then paid homage to William and the two, with one hundred knights given them by the king plus each leader's own personal retinue, set off to conquer