tsar of the Second Bulgarian empire from 1186 to 1196, during one of the most brilliant periods of the restored Bulgarian nation. He and his brother Peter II were founders of the Asen dynasty, which survived until the latter half of the 13th century.

Asen was a descendant of landowners and boyars from Tŭrnovo whose family name was Belgun. In 1186, after a violent dispute with the Byzantine emperor Isaac II Angelus, Asen and Peter led a popular rising of Vlachs and Bulgars and proclaimed their independence from the Byzantines. Asen was crowned tsar as Ivan Asen I at Tŭrnovo, and Peter became ruler of the eastern half of the kingdom, with his residence at Preslav (now Veliki Preslav). The brothers invaded Thrace but were defeated, withdrew to the north, and, in alliance with the Kumans, conquered northern Bulgaria. In 1187 they checked the Byzantine army in Thrace; and, in the armistice that followed, their younger brother Kaloyan was sent as hostage to Constantinople (now Istanbul). He escaped, however, and war broke out, to continue intermittently until the Byzantine forces were thoroughly defeated in 1196. Later in that year, Ivan Asen was killed by one of his boyars, Ivanko, who seized power at Tŭrnovo but soon had to seek refuge in Constantinople. Asen's brother Peter ascended the throne as Peter II but was killed by the boyars in 1197. Kaloyan was then crowned tsar, reigning from 1197 to 1207.