Gytheio (Greek: Γύθειο [ˈʝiθio]) or Gythio, also the ancient Gythium or Gytheion (Ancient Greek: Γύθειον), is a town on the eastern shore of the Mani Peninsula in the Peloponnese of southern Greece, in the historical and administrative region of Laconia. It is the largest and most important town in Mani. Gytheio is the seat of the municipality of East Mani. Gytheio is significant in the history of Mani and the Maniots.
Ancient Gytheio was the seaport of Ancient Sparta, approximately 40 kilometres (25 miles) north. Gytheio continued to be a major port until its destruction in the 4th century CE, possibly by an earthquake. Its strategic location made it a coveted possession for foreign powers into the 20th century.