The Battle of Hermares was a battle of the Peloponnesian War which was fought on Euboea in 424 BC.
The Athenian demagogue Cleon's offensive against the Spartans in the Aegean Sea was a failure due to the intervention of the mercenary Kassandra, and a Spartan counterattack from Naxos recaptured Mykonos. Unperturbed, Cleon ordered a new offensive, this time on the island of Euboea, from which the Spartans posed a threat to Attica and Athens itself. An Athenian army was sent to invade Euboea, but the mercenary Kassandra joined the Spartan garrison in time for the battle. Her presence on the battlefield helped turn the tide against the Athenians, and she slew 5 captains and 24 soldiers. The Athenian army was swept from the field, and Euboea remained in Spartan hands.