The Tsardom of Bulgaria (Bulgarian: Царство България, romanizedTsarstvo Bŭlgariya), also referred to as the Third Bulgarian Tsardom (Bulgarian: Трето Българско Царство, romanizedTreto Bŭlgarsko Tsarstvo), was a constitutional monarchy in Eastern and 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. Ferdinand was crowned a 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).

The state was almost constantly 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 once again went to war for national unification as part of the Second World War, and once again found itself on the losing side, until it switched sides to the Allies in 1944. In 1946, the monarchy was abolished, its final Tsar was sent into exile and the Kingdom was replaced by the People's Republic of Bulgaria.