Poseidonia (Greek: Ποσειδωνία, romanized: Posidonía, named after Poseidon) is a village and a former municipality on the island of Syros, in the Cyclades, Greece.

[2] The population was 4,114 inhabitants at the 2021 census, and the land area is 23.705 km².

[3] The municipal unit shares the island of Syros with the municipal units of Ano Syros and Ermoupoli.

It is also known as Dellagrazia (name taken from a small church of Maria della Grazia).

This South Aegean location article is a stub.