Walter V, Count of Brienne

Walter V of Brienne (French: Gautier; c. 1275 – 15 March 1311) was Duke of Athens from 1308 until his death. The only son of Hugh of Brienne and Isabella de la Roche, he was the heir to large estates in France, the Kingdom of Naples, and the Peloponnese. Between 1287 and 1296/97, he was held in custody in the Sicilian castle of Augusta as security for his father's ransom to the Aragonese admiral Roger of Lauria. Following his father's death in battle against Lauria in 1296, Walter succeeded to the County of Brienne in France, and the counties of Lecce and Conversano in southern Italy. Though released soon afterwards, he was captured during a Neapolitan campaign in Sicily in 1299, remaining in captivity until the Treaty of Caltabellotta in 1302.

Walter settled in France and married Joanna of Châtillon. After his cousin, Duke Guy II of Athens, died childless in 1308, Walter asserted his claim to the duchy. Their cousin, Eschiva of Ibelin, also asserted her right, but the High Court of Achaea ruled in Walter's favour, and he arrived in Athens in 1309. John II Doukas, the Greek ruler of Thessaly, formed an alliance against him with the Byzantine Empire and the Despotate of Epirus. To counter this, Walter engaged the Catalan Company, a band of mercenaries, to invade Thessaly. The Catalans routed John, yet Walter denied them their wages; they rose in rebellion, and Walter gathered a substantial force from Frankish Greece. At the Battle of Halmyros, however, the Catalans crushed his army. Walter fell in the fighting, and the Catalans seized control of the Duchy of Athens.