Eion (; gen.: Ἠϊόνος) was an ancient Greek Eretrian colony in Thracian Macedonia. It sits at the mouth of the Strymon River which flows into the Aegean from the interior of Thrace. It is referred to in Thucydides' History of the Peloponnesian War as a place of considerable strategic importance to the Athenians during the Peloponnesian War.