The Treaty of Constantinople establishes the Septinsular Republic, the first autonomous Greek state since the Fall of the Byzantine Empire. (Under the Old Style calendar then still in use in the Ottoman Empire, the treaty was signed on 21 March.)

The Treaty of Constantinople of 2 April [O.S. 21 March] 1800 was concluded between the Ottoman Empire and the Russian Empire, and heralded the creation of the Septinsular Republic, the first autonomous Greek state since the fall of the Byzantine Empire in 1453.