Bulgarian (български език, pronounced: [ˈbɤ̞ɫɡɐrski ɛˈzik]) is an Indo-European language, a member of the Southern branch of the Slavic language family.

Bulgarian, along with the closely related Macedonian language, has several characteristics that set it apart from all other Slavic languages such as the elimination of case declension, the development of a suffixed definite article (see Balkan linguistic union), the lack of a verb infinitive, and the retention and further development of the Proto-Slavic verb system. Various evidential verb forms exist to express unwitnessed, retold, and doubtful action. Estimates of the number of people around the world who speak Bulgarian fluently range from about 9 million to 12 million.

The development of the Bulgarian language may be divided into several periods.

Bulgarian was the first "Slavic" language attested in writing. As Slavic linguistic unity lasted into late antiquity, in the oldest manuscripts this language was initially referred to as языкъ словяньскъ, "the Slavic language". In the Middle Bulgarian period this name was gradually replaced by the name языкъ блъгарьскъ, the "Bulgarian language". In some cases, the name языкъ блъгарьскъ was used not only with regard to the contemporary Middle Bulgarian language of the copyist but also to the period of Old Bulgarian. A most notable example of anachronism is the Service of St. Cyril from Skopje (Скопски миней), a 13th-century Middle Bulgarian manuscript from northern Macedonia according to which St. Cyril preached with "Bulgarian" books among the Moravian Slavs. The first mention of the language as the "Bulgarian language" instead of the "Slavonic language" comes in the work of the Greek clergy of the Bulgarian Archbishopric of Ohrid in the 11th century, for example in the Greek hagiography of Saint Clement of Ohrid by Theophylact of Ohrid (late 11th century).