Peloponnesos (Greek: Πελοπόννησος, "Pelops' Island", sometime Latinized as Peloponnesus or Anglicized as The Peloponnese) is a large peninsula in Greece, forming the part of the country south of the Isthmus of Corinth.

Its name derives from the Ancient Greek Mythological Hero, 'Pelops', who supposedly conquered the entire region. Of the name, Peloponnesos, 'Pelops' refers to this hero and 'nesos' refers to island, therefore the name means the Island of Pelops. However, Peloponnesos only became a true island with the creation of the Corinthian Canal in 1893. In 2004 Peloponnesos gained a second connection to the mainland, with the completion of the Rio-Antirio bridge.

"The Peloponnese" is sometimes used as shorthand for the Peloponnesian League.

Though "Peloponnesos" is used to refer to the entire peninsula, the periphery with that name includes only part of that landmass.

In medieval times it was called the Morea, because the Crusaders found it densely planted with mulberry trees (Greek: moreai) used by the flourishing Byzantine silk industry.

Peloponnesos is a periphery of Greece, consisting of 5 prefectures (Greek: νομοι):

• Arcadia
• Argolis
• Corinth
• Laconia
• Messinia

The prefectures of Achaea and Ilia are also part of the peninsula, but are part of the West Greece periphery. A small part of the peninsula belongs to the periphery of Attica.