The Battle of Nysonion was a battle of the Peloponnesian War which was fought between Sparta and Athens in Megaris in 422 BC.
The Spartans' successes in Argolis and Locris motivated a third push on Athenian borders in Megaris, with an army from Corinthia being dispatched to capture Megara. The mercenary Kassandra joined the Spartans after assassinating the pro-Athenian ruler Pylartes the Gilded Shark, while the Athenians hired Neleos the Charming as their champion. In the ensuing battle, Kassandra slew Neleos, 23 Athenian soldiers, and 8 captains. Her feats enabled the Spartans to rout the Athenians and reoccupy Megaris.