The Tsardom of Bulgaria (Bulgarian: Царство България, romanized: Tsarstvo Bŭlgariya), also known as the Third Bulgarian Tsardom (Bulgarian: Трето Българско Царство, romanized: Treto Bŭlgarsko Tsarstvo), usually known in English as the Kingdom of Bulgaria, or simply Bulgaria, was a constitutional monarchy in Southeastern Europe, which was established on 5 October [O.S. 22 September] 1908, when the Bulgarian state was raised from a principality to a tsardom.

Prince Ferdinand, founder of the royal family, was crowned as tsar at the Declaration of Independence, mainly because of his military plans and for seeking options for unification of all lands in the Balkans region with an ethnic Bulgarian majority (lands that had been seized from Bulgaria and given to the Ottoman Empire in the Treaty of Berlin). He and his successors were reckoned as kings internationally.

The state was frequently at war throughout its existence, lending to its nickname as "the Balkan Prussia". For several years, Bulgaria mobilized an army of more than 1 million people from its population of about 5 million, and in the 1910s it engaged in three wars — the First and Second Balkan Wars, and the First World War. Following the First World War the Bulgarian army was disbanded and forbidden to exist by the Allied Powers, and all plans for national unification of the Bulgarian lands failed.

Less than two decades later Bulgaria entered World War II on the side of the Axis powers and once again found itself on the losing side until it switched sides to the Allies in September 1944. In 1946, the monarchy was abolished, its final tsar was sent into exile, and the tsardom was replaced by the People's Republic of Bulgaria.

Despite the establishment of the Principality of Bulgaria (subject to Ottoman suzerainty) in 1878, and the subsequent Bulgarian control over Eastern Rumelia after 1885, there was still a substantial Bulgarian population in the Balkans living under Ottoman rule, particularly in Macedonia. To complicate matters, Serbia and Greece too made claims over parts of Macedonia, while Serbia, as a Slavic nation, also considered Macedonian Slavs as belonging to the Serbian nation. Thus began a three-sided struggle for control of these areas which lasted until World War I. In 1903 there was a Bulgarian insurrection in Ottoman Macedonia.

In 1908 Ferdinand used the struggles among the Great Powers to declare Bulgaria an independent kingdom with himself as Tsar. He did this on 5 October (though celebrated on 22 September, as Bulgaria remained officially on the Julian Calendar until 1916) in the St Forty Martyrs Church in Veliko Tarnovo. Even before then, however, Bulgaria had only acknowledged the overlordship of the sultan formally. Since 1878, Bulgaria had had its own constitution, flag, and anthem, and conducted a separate foreign policy.

Ferdinand took the Bulgarian title "Tsar" in honor of the rulers of the First and Second Bulgarian Empires. However, while previous Bulgarian "tsars" were reckoned as emperors, Ferdinand and his successors were called "kings" outside Bulgaria. The Tarnovo Constitution was retained, with the word "tsar" replacing the word "prince" or ''knyaz''.

In 1911 the Nationalist Prime Minister Ivan Geshov set about allying with Greece and Serbia, and the three allies agreed to put aside their rivalries to plan a joint attack on the Ottomans.