Gytheio (Greek: Γύθειο) or Gythio, also the ancient Gythium or Gytheion (Ancient Greek: Γύθειον), is a town on the eastern shore of the Mani Peninsula, 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. The municipal unit has an area of 197.313 km2. It was the seaport of Sparta, some 40 kilometres (25 miles) north. Gytheio is the site of ancient Cranae, a tiny island where according to legend Paris of Troy and Helen from Sparta spent their first night together before departing for Troy, thus igniting the Trojan War.
Gytheio used to be an important port until it was destroyed in 4th century AD, possibly by an earthquake. Even thereafter its strategic location gave Gytheio a significant role in Maniot history. Today it is the largest and most important town in Mani. It is also the seat of the municipality of East Mani.