The battle of the Cephisus River (c.352) was the second in a series of defeats suffered by the Phocian leader Phayllus during a failed invasion of Boeotia (Third Sacred War).
Phayllus became the Phocian leader after his brother Onomarchus was killed at the battle of the Crocus Field in Thessaly (353 BC). Almost half of the Phocian army was destroyed in that battle, but Phayllus was soon able to recruit fresh troops. He was also helped by the arrival of 2,000 men under the defeated tyrants of Pherae and troops sent by his allies (1,000 from Sparta, 2,000 from Achaea and 5,000 infantry and 400 cavalry from Athens).
Phayllus used his new army to carry out an unsuccessful invasion of Boeotia. His first target was the city of Orchomenus, but he suffered a defeat in battle near the city.
Next came a costly defeat on the Cephisus River. Diodorus provides no details of the battle itself, but records the Phocian losses as 500 dead and 400 prisoners.
The Cephisus River rises on the northern slopes of Mount Parnassus, then flows east into Lake Copais, and from there across Boeotia, before turning north to reach the sea. Diodorus doesn't say which part of the river the battle was fought at, but he does report a third battle a few days later at Coroneia. This might suggest that the Phocians moved east along the northern shores of Lake Copais after the defeat at Orchomenus, suffered their second defeat on the stretch of the river between the lake and the sea, and then attempted to return home along the southern side of the lake, where they suffered their third defeat.