Katharevousa (Greek: Καθαρεύουσα, pronounced [kaθaˈrevusa], literally "purifying [language]") is a conservative form of the Modern Greek language conceived in the late 18th century as both a literary language and a compromise between Ancient Greek and the contemporary vernacular, Demotic Greek. Originally, it was widely used for both literary and official purposes, though sparingly in daily language. In the 20th century, it was increasingly adopted for official and formal purposes, until minister of education Georgios Rallis made Demotic Greek the official language of Greece in 1976, and in 1982 Prime Minister Andreas Papandreou abolished the polytonic system of writing for both Demotic and Katharevousa.

Katharevousa was conceived by the intellectual and revolutionary leader Adamantios Korais (1748–1833). A graduate of the University of Montpellier, Korais spent most of his life as an expatriate in Paris. As a classical scholar credited with both laying the foundations of Modern Greek literature and a major figure in the Greek Enlightenment, he was repelled by the Byzantine and subsequent influence on Greek society, and was a fierce critic of the clergy and their alleged subservience to the Ottoman Empire. He held that education was a prerequisite to Greek liberation.

Part of Katharevousa's purpose was to serve as a compromise solution for the struggle between the "archaists" demanding full reversion to archaic Greek, and the "modernists".