The Principality of Bulgaria (Bulgarian: Княжество България, Knyazhestvo Balgariya) was a de facto independent, and de jure vassal state under the suzerainty of the Ottoman Empire. It was established by the Treaty of Berlin in 1878.

After the Russo-Turkish War ended with a Russian victory, the Treaty of San Stefano was signed by Russia and the Ottoman Empire on 3 March 1878. Under this, a large Bulgarian vassal state was agreed to, which was significantly larger: its lands encompassed nearly all ethnic Bulgarians in the Balkans, and included most of Moesia, Thrace and Macedonia, stretching from the Black Sea to the Aegean. However, the United Kingdom and Austria-Hungary were against the establishment of such a large Russian client state in the Balkans, fearing it would shift the balance of power in the Mediterranean. Due to this, the great powers convened and signed the Treaty of Berlin, superseding the Treaty of San Stefano, which never went into effect. This created a much smaller principality, alongside an autonomous Eastern Rumelia within the Ottoman Empire.

Although an Ottoman vassal, Bulgaria only acknowledged the authority of the Sublime Porte in a formal way. It had its own Constitution, flag and anthem, and conducted its own foreign policy. In 1885, a bloodless revolution resulted in Eastern Rumelia being de facto annexed by Bulgaria, which the Ottoman Empire accepted with the Tophane Agreement. On 5 October 1908, Bulgaria declared its independence as the Kingdom of Bulgaria.

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