The Peloponnese, Peloponnesus, or Morea, is a peninsula and geographic region in Southern Greece, and the southernmost region of the Balkans. It is connected to the central part of the country by the Isthmus of Corinth land bridge, which separates the Gulf of Corinth from the Saronic Gulf. From the late Middle Ages until the 19th century, the peninsula was known as the Morea, a name still in colloquial use in its demotic form.

The peninsula is divided among three administrative regions: most belongs to the Peloponnese region, with smaller parts belonging to the West Greece and Attica regions.

The Peloponnese is a peninsula located at the southern tip of the mainland, 21,549.6 square kilometres (8,320.3 sq mi) in area, and constitutes the southernmost part of mainland Greece. It is connected to the mainland by the Isthmus of Corinth, where the Corinth Canal was constructed in 1893. However, it is also connected to the mainland by several bridges across the canal, including two submersible bridges at the north and the south end. Near the northern tip of the peninsula, there is another bridge, the Rio–Antirrio bridge (completed 2004). The peninsula has a mountainous interior, although extensive lowlands are also found in the west, in the Evrotas Valley in the south, and in the Argolid Peninsula in the northeast. The Peloponnese possesses four south-pointing peninsulas: the Messenian, the Mani, the Cape Malea, and the Argolid in the far northeast of the Peloponnese. The Messenian Gulf, Laconian Gulf, and Argolic Gulf separate these peninsulas. Moreover, Mount Taygetus in the south is the highest mountain in the Peloponnese, at 2,407 metres (7,897 ft). Οther important mountains include Cyllene in the northeast (2,376 m or 7,795 ft), Aroania in the north (2,355 m or 7,726 ft), Erymanthos (2,224 m or 7,297 ft) and Panachaikon in the northwest (1,926 m or 6,319 ft), Mainalon in the center (1,981 m or 6,499 ft), and Parnon in the southeast (1,935 m or 6,348 ft). The entire peninsula is earthquake-prone and has suffered many earthquakes in the past.

The longest river is the Alfeios in the west (110 km or 68 mi), followed by the Evrotas in the south (82 km or 51 mi), and also the Pineios, also in the west (70 km or 43 mi). The Peloponnese, with its indented coasts, is home to numerous spectacular beaches, which are a major tourist draw.

Two groups of islands lie off the Peloponnesian coast: the Argo-Saronic Islands to the east, and the Ionian to the west. The island of Kythira, to the south of the Peloponnese, is considered to be part of the Ionian Islands. The island of Elafonisos used to be part of the peninsula but was separated in the aftermath of the 365 Crete earthquake.

Since antiquity, and continuing to the present day, the Peloponnese has been divided into seven major regions: Achaea (north), Corinthia (northeast), Argolis (east), Arcadia (center), Laconia (southeast), Messenia (southwest), and Elis (west). Each of these regions is headed by a city, with the largest being Patras (pop. 170,000) in Achaia, followed by Kalamata (pop. 55,000) in Messenia.

The Peloponnese for the most part enjoys a hot-summer Mediterranean climate (Köppen climate classification: Csa), while the Gulf of Corinth has a hot semi-arid climate (Köppen climate classification: BSh). Rainfall is higher on the west coast, while the east of the peninsula is significantly drier. Average annual temperatures can reach up to 20.3 °C (68.5 °F) while summer highs reach over 36.0 °C (96.8 °F) in Sparta, within the Evrotas Valley. On 27 June 2007, Monemvasia registered a staggering minimum temperature of 35.9°C, which is the highest minimum temperature ever recorded in mainland Greece and Continental Europe.

The peninsula has been inhabited since prehistoric times. Its modern name derives from ancient Greek mythology, specifically the legend of the hero Pelops, who was said to have conquered the entire region. The name Peloponnesos means Island or Peninsula (archaic meaning of the word nesos) of Pelops.