Katharevousa (Greek: Καθαρεύουσα, [kaθaˈrevusa], lit. "puristic [language]"), is a conservative form of the Modern Greek language conceived in the early 19th century as a compromise between Ancient Greek and Dimotiki of the time. Originally, it was widely used both for literary and official purposes, though seldom in daily language. In the 20th century, it was increasingly just for official and formal purposes, until Dimotiki became the official language of Greece in 1976 and Andreas Papandreou abolished the polytonic system of writing in 1981.

Katharevousa was conceived by the intellectual and revolutionary leader Adamantios Korais. A graduate of the University of Montpellier, Korais spent most of his life as an expatriate in Paris. Being a classical scholar, he was repelled by the Byzantine and later 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 its purpose was to mediate the struggle between the "archaists" favouring full reversion to archaic forms, and the "modernists". The name "Katharevousa" implies a pure form of Greek as it might hypothetically have evolved from ancient Greek without external influences.