The 1878 Treaty of San Stefano (Russian: Сан-Стефанский мир; Peace of San-Stefano, Сан-Стефанский мирный договор; Peace treaty of San-Stefano, Turkish: Ayastefanos Muahedesi or Ayastefanos Antlaşması) was a treaty between the Russian and Ottoman empires at the conclusion of the Russo-Turkish War of 1877–1878. It was signed at San Stefano, then a village west of Constantinople (present-day Istanbul), on 3 March  [O.S. 19 February] 1878 by Count Nicholas Pavlovich Ignatiev and Aleksandr Nelidov on behalf of the Russian Empire and by Foreign Minister Saffet Pasha and Ambassador to Germany Sadullah Bey on behalf of the Ottoman Empire.

According to the official Russian position, by signing the treaty, Russia had never intended anything more than a temporary rough draft, so as to enable a final settlement with the other Great Powers.

The treaty provided for the establishment of an autonomous Principality of Bulgaria following almost 500 years of Ottoman rule in the Bulgarian lands. Bulgarians celebrate the day the treaty was signed, 3 March  [O.S. 19 February] 1878, as Liberation Day. However, the enlarged Bulgaria envisioned by the treaty alarmed neighboring states as well as France and Great Britain. As a result, the enlargement was never implemented, being superseded by the Treaty of Berlin following the Congress of the same name that took place three months later.