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.

The battle was a decisive event in the history of Frankish Greece; almost the entire Frankish elite of Athens and its vassal states lay dead on the field or in captivity, and when the Catalans moved onto the lands of the Duchy, there was scant resistance. The Greek inhabitants of Livadeia immediately surrendered their strongly fortified town, for which they were rewarded with the rights of Frankish citizens. Thebes, the capital of the Duchy, was abandoned by many of its inhabitants, who fled to the Venetian stronghold of Negroponte, and was plundered by the Catalan troops. Finally, Athens was surrendered to the victors by Walter's widow, Joanna of Châtillon. All of Attica and Boeotia passed peacefully into the hands of the Catalans. The Catalans divided the territory of the Duchy among themselves. The decimation of the previous feudal aristocracy allowed the Catalans to take possession relatively easily, in many cases marrying the widows and mothers of the very men they had slain in Halmyros. The Catalans' Turkish allies, however, refused the offer to settle in the Duchy. The Turks of Halil took their share of the booty and headed for Asia Minor, only to be attacked and almost annihilated by a joint Byzantine and Genoese force as they tried to cross the Dardanelles a few months later.