Walter V of Brienne (French: Gautier; c.1275 15 March 1311) was Duke of Athens from 1308 until his death. Being the only son of Hugh of Brienne and Isabella de la Roche, Walter was the heir to large estates in France, the Kingdom of Naples, and the Peloponnese. He was held in custody in the Sicilian castle of Augusta between 1287 and 1296 or 1297 to secure the payment of his father's ransom to the Aragonese admiral Roger of Lauria. When his father died fighting against Lauria in 1296, Walter inherited the County of Brienne in France, and the counties of Lecce and Conversano in southern Italy. He was released, but he was captured during a Neapolitan invasion of Sicily in 1299. His second captivity lasted until the Treaty of Caltabellotta in 1302.

Walter settled in France and married Joanna of Chtillon. After his cousin Duke Guy II of Athens died childless in 1308, Walter laid claim to the Duchy of Athens. Their cousin Eschiva of Ibelin also claimed the duchy, but the High Court of Achaea passed a judgement in Walter's favor. Walter came to Athens in 1309. John II Doukas, the Greek lord of Thessaly, made an alliance against him with the Byzantine Empire and the Despotate of Epirus. Walter hired the Catalan Company, a group of mercenaries, to invade Thessaly. The Catalans defeated John, but Walter refused to pay their wages. After the Catalans rose up in open rebellion, Walter assembled a large army from Frankish Greece, but the Catalans inflicted a crushing defeat on the Franks in the Battle of Halmyros. Walter died in the battlefield and the Catalans occupied the Duchy of Athens.



The Battle of Halmyros, known by earlier scholars as the Battle of the Cephissus or Battle of Orchomenos, was fought on 15 March 1311, between the forces of the Frankish Duchy of Athens and its vassals under Walter of Brienne against the mercenaries of the Catalan Company, resulting in a decisive victory for the mercenaries.

Engaged in conflict with their original employers, the Byzantine Empire, the Catalan Company had traversed the southern Balkans and arrived in southern Greece in 1309. The new Duke of Athens, Walter of Brienne, hired them to attack the Greek ruler of neighbouring Thessaly. Although the Catalans conquered much of the region for him, Walter refused to pay them and prepared to forcibly expel them from their gains. The two armies met at Halmyros in southern Thessaly (or at the Boeotic Cephissus, near Orchomenos, according to an earlier interpretation). The Catalans were considerably outnumbered and weakened by the reluctance of their Turkish auxiliaries to fight. The Company did have the advantage of selecting the battleground, positioning themselves behind marshy terrain, which they further inundated. On the Athenian side, many of the most important lords of Frankish Greece were present and Walter, a prideful man and confident in the prowess of his heavy cavalry, proceeded to charge headlong against the Catalan line. The marsh impeded the Frankish attack and the Catalan infantry stood firm. The Turks re-joined the Company and the Frankish army was routed; Walter and almost the entire knighthood of his realm fell in the field. As a result of the battle, the Catalans took over the leaderless Duchy of Athens; they ruled that part of Greece until the 1380s.