Posideion (in ancient Greek, Ποσίδειον), is the name of an ancient Greek city of the Chalcidice peninsula.
Belonged to the League of Delos since it appears in the record of taxation of Athens of the year 422/1 a. C., where it belonged to the district of Thrace and had to pay a phoros of 1000 drachmas.
Its exact location is unknown, but it has been suggested that it may be related to the Poseidon sanctuary that Herodotus mentions among the cities of Argilo and Estagira. Another possibility is to locate it on a promontory located 4 km from Mende.
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