The occupation of Istanbul, the capital of the Ottoman Empire, by British, French, Italian, and Greek forces, took place in accordance with the Armistice of Mudros, which ended Ottoman participation in the First World War. The first French troops entered the city on 12 November 1918, followed by British troops the next day. 1918 saw the first time the city had changed hands since the Fall of Constantinople in 1453. Along with the Occupation of Smyrna, it spurred the establishment of the Turkish National Movement, leading to the Turkish War of Independence.

Allied troops occupied zones based on the existing divisions of Istanbul and set up an Allied military administration early in December 1918. The occupation had two stages: the initial phase in accordance with the Armistice gave way in 1920 to a more formal arrangement under the Treaty of Sèvres. Ultimately, the Treaty of Lausanne, signed on 24 July 1923, led to the end of the occupation. The last troops of the Allies departed from the city on 4 October 1923, and the first troops of the Ankara government, commanded by Şükrü Naili Pasha (3rd Corps), entered the city with a ceremony on 6 October 1923, which has been marked as the Liberation Day of Istanbul and is commemorated every year on its anniversary.