The Battle of Cynossema was a naval battle in the Hellespont in 411 BC between Athens and Sparta, around the same time the Athenian democracy was overthrown in favour of a short-lived oligarchy.

The Peloponnesian fleet, led by Sparta, was stationed at Miletus waiting for the Phoenician fleet (which, unknown to them, was being held up by the Athenian Alcibiades elsewhere). The Spartan admiral Mindarus gathered seventy-three ships and sailed to Chios, prompting the Athenian admiral Thrasylus to sail with fifty-five ships from Samos. Thrasybulus joined him with five ships, and seven other allied ships joined them. When Mindarus sailed from Chios, he was joined by ships from Abydos, raising his total to eighty-six. They defeated a group of eighteen Athenian ships from Sestos, capturing or destroying four of them. The other Athenian ships, now numbering seventy-six, sailed towards the Hellespont when they learned of the movements of the Peloponnesians.

The Peloponnesian ships were lined up with the Syracusans on the right wing, and the Spartans on the left. The Athenian line was formed with Thrasylus on the right and Thrasybulus on the left. In their attempts to outflank each other, the lines eventually extended to Cynossema . The Spartans were at first victorious, having pushed through the centre of the Athenian line. They assumed they had won the battle and began chasing individual Athenian ships, but Thrasybulus, ignoring the Syracusans for the moment, reorganized and soon routed the Spartan ships, which had moved out of formation. The Spartans retreated, and the Syracusans followed. The Athenians captured 21 ships, but the others were able to escape to Abydos. The Athenians lost 15 ships.

The victory prompted the oligarchy in Athens to be overthrown, and the democracy restored.