The Megali Idea (Greek: Μεγάλη Ιδέα, romanized: Megáli Idéa, lit. 'Great Idea') is a nationalist and irredentist concept that expresses the goal of reviving the Byzantine Empire, by establishing a Greek state, which would include the large Greek populations that were still under Ottoman rule after the end of the Greek War of Independence (1821–1829) and all the regions that had large Greek populations (parts of the southern Balkans, Anatolia and Cyprus).

The term appeared for the first time during the debates of Prime Minister Ioannis Kolettis with King Otto that preceded the promulgation of the 1844 constitution. It came to dominate foreign relations and played a significant role in domestic politics for much of the first century of Greek independence. The expression was new in 1844 but the concept had roots in the Greek popular psyche, which long had hopes of liberation from Ottoman rule and restoration of the Byzantine Empire.

Πάλι με χρόνια με καιρούς,

(Once more, as years and time go by, once more they shall be ours).

The Megali Idea implies establishing a Greek state, which would be a territory encompassing mostly the former Byzantine lands from the Ionian Sea in the west to Anatolia and the Black Sea to the east and from Thrace, Macedonia and Epirus in the north to Crete and Cyprus to the south. This new state would have Constantinople as its capital: it would be the "Greece of Two Continents and Five Seas" (Europe and Asia, the Ionian, Aegean, Marmara, Black and Libyan Seas). If realized, this would expand modern Greece to roughly the same size and extent as the later Byzantine Empire, after its restoration in 1261 AD.

The Megali Idea dominated foreign policy and domestic politics of Greece from the War of Independence in the 1820s through the Balkan wars in the beginning of the 20th century. It started to fade after the Greco-Turkish War (1919–1922), followed by the population exchange between Greece and Turkey in 1923. Despite the end of the Megali Idea project in 1922, by then the Greek state had expanded four times, either through military conquest or diplomacy (often with British support). After the creation of Greece in 1830, it acquired the Ionian Islands (Treaty of London, 1864), Thessaly (Convention of Constantinople (1881)), Macedonia, Crete, (southern) Epirus and the Eastern Aegean Islands (Treaty of Bucharest), and Western Thrace (Treaty of Neuilly, 1920). The Dodecanese were acquired after the Second World War (Treaty of Peace with Italy, 1947).

A related concept is Enosis.

The Byzantine Empire was Eastern Roman in origin and was called the "Roman Empire" by its inhabitants, though often not by the Latin West, which regarded it as Greek. After its fall, Hieronymus Wolf popularized the usage of "Byzantium". An informal cultural division had existed within the Roman Empire for centuries. Although Latin was the official language of the empire, Greek was the lingua franca in the East and was regularly used alongside Latin in official business. The division of the Empire following the death of Emperor Theodosius in AD 395 added a formal political layer to the informal cultural division. Following the adoption of Christianity in the 4th century, Greek was also the dominant liturgical language, as exemplified by the fact that the New Testament was written in Greek. These factors ultimately led to Greek replacing Latin as the official language in AD 620.