The battle of Hermeum (354 or 353 BC) was a Phocian victory over the Boeotians (Third Sacred War), which followed a brief Phocian intervention in Thessaly that saw them inflict two rare battlefield defeats on Philip II.
After earlier events of 354 any Phocian successes seemed unlikely. Their leader Philomelus had been defeated and killed by the Boeotians at the battle of Neon, and his brother Onomarchus had only just been able to convince his people to keep fighting. Onomarchus had then sent an army commanded by his brother Phayllus north into Thessaly to help Lycophron, tyrant of Pherae, but Philip II of Macedon had defeated that army.
Onomarchus restored the Phocian position. First he led the main army into Thessaly, where he inflicted two rare battlefield defeats on Philip. Sadly these battles are terribly badly documented. After these defeats Philip withdrew into Macedon.
Onomarchus took his army south into Boeotian. Diodorus (16.35.3) records that he defeated the Boeotians in battle, and captured Coronea, but gives no other details.
Aristotle provides a little more detail (Nic. Eth. 1116b). The battle is located as being fought at the temple of Hermes (the Hermeum). The Boeotians had a mixed force of citizens and mercenaries, but were probably outnumbered, as Aristotle was making a point about the reluctance of mercenaries to stand and fight when at a disadvantage in numbers and equipment.
This was a short-lived success. Philip quickly recovered from his two defeats, and led his army back to Thessaly to attack Lycophron. Onomarchus headed north in response, but was killed at the battle of the Crocus Field.