Gythio, the ancient Gytheion or Gythium, is a town located in Laconia, Peloponnese, Greece. It was the seaport of the city-state of Sparta about 40 km (25 miles) north.

Gythio was an port of great importance until its destruction in the 4th century AD, most likely by an earthquake. Today is the most impotrant and the largest town in Mani. Situate in the notheast corner of Mani and on the nothwest end of the Laconian Gulf. Gythio has been built in one of the most fertile areas in Mani, on a hill called Koumaros or Laryssio.

The founders of ancient Gytheion were the demigod Hercules & the god Apollo, who both appeared often on its coin and Castor & Pollux the origin of which may point to the Phoenician influence of the traders from Tyre who we know would visit the Laconian shores at a very early period.