Gytheio (Greek: Γύθειο, [ˈʝiθio]), the ancient Gythium or Gytheion (Ancient Greek: Γύθειον), is a town and a former municipality in Laconia, Peloponnese, Greece. Since the 2011 local government reform it is part of the municipality East Mani, of which it is a municipal unit. It was the seaport of Sparta, some 40 km north. Gytheio used to be an important port until it was destroyed in 4th century AD, possibly by an earthquake. Today it is the largest and most important town in Mani. It is also the seat of the municipality of East Mani.

Gytheio is located in the north-east corner of Mani. It lies on the north-western end of the Laconian Gulf. Gytheio was built on a hill called Koumaros or Laryssio in one of the most fertile areas in Mani, near the mouth of the Gythium River, which is usually dry and has been given the nickname of "Xerias", meaning 'dry' (Today most of the "Xerias" dry river is covered by "Ermou Avenue"). Further north-east is the delta of the Evrotas River. Gytheio is built on hilly ground overlooking the Laconian Gulf. Offshore of Gytheio are several small islands, the most important of these being Cranae, which is connected to the mainland by a causeway. Gytheio is only 40 km southeast of Sparta.