Poseidonia (, named after Poseidon) is a village and a former municipality on the island of Syros, in the Cyclades, Greece. Since the 2011 local government reform it is part of the municipality Syros-Ermoupoli, of which it is a municipal unit. The population was 4,114 inhabitants at the 2021 census, and the land area is 23.705 kmò. 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).