Poseidonia, formerly known as Delagratzia, is a settlement of Syros. It is located in the wide bay of Finikas and is 10 km from the capital Ermoupolis. It has been for many years the resort of the rich Syrians.

The first name comes from the ancient Poseidonia which is supposed to be there and was dedicated to the god Posidon. The second name comes from the small Catholic church Madonna de la Gratsia (Virgin of Grace) located next to the small cemetery of the settlement.

The neoclassical countryside villas with towers and large gardens with pine and palm trees. Built in the late 19th century, are remarkable in the settlement. The most famous is Tsiproina villa (former mayor of Ermoupolis), which has been characterized as a historic and preserved monument. Today, the building is used for various kinds of events.

Near the settlement there are the beaches of Fetouri, Voulgari, Agathopes and Komitos. They attract many holidaymakers during the summer months. In Agathopes there is a large population of sea lilies.