The Serbo-Bulgarian War or the Serbian–Bulgarian War was a war between the Kingdom of Serbia and Principality of Bulgaria that erupted on 14 November [O.S. 2 November] 1885 and lasted until 28 November [O.S. 16 November] 1885. Despite Bulgaria being a vassal state of the Ottoman Empire, the Ottomans did not intervene in the war. Serbia initiated the war but was decisively defeated. Austria-Hungary demanded for Bulgaria to stop its invasion, and a truce resulted. The final peace was signed on 3 March [O.S. 19 February] 1886 in Bucharest. The old boundaries were not changed. As a result of the war, European powers acknowledged the act of Unification of Bulgaria which happened on 18 September [O.S. 6 September] 1885.

The Kingdom of Serbia was a country located in the Balkans which was created when the ruler of the Principality of Serbia, Milan I, was proclaimed king in 1882. Since 1817, the Principality was ruled by the Obrenović dynasty. The Principality, under the suzerainty of the Ottoman Empire, de facto achieved full independence when the very last Ottoman troops left Belgrade in 1867. The Congress of Berlin in 1878 recognized the formal independence of the Principality of Serbia, and in its composition Nišava, Pirot, Toplica and Vranje districts entered the South part of Serbia.