BORIS I. (?-9o3), Bulgarian tsar, succeeded his father Pres sian in 853. The early years of his rule were occupied by wars in Central Europe. His alliance with Ludwig the German caused the Byzantine emperor, Michael III., to make overtures to Boris with a view to an alliance, to be sealed by Boris' conversion to the Orthodox faith. A treaty was concluded in 864, and Boris was baptized in the following year. In 866, however, he ap proached the pope, Nicholas I., in the hope of securing from the Western Church the promise of an independent national patri archate for Bulgaria. The pope sent Latin clergy to Bulgaria and promised to appoint an independent head of the national church; but on his failing to fulfil his promise, Boris finally decided for the Byzantine Church at the Council of Constanti nople (87o). The Bulgarian nation accepted the Orthodox faith and numerous monasteries sprang up throughout Bulgaria. Into one of these Boris retired to spend his last years (880, after abdicating in favour of his eldest son Vladimir; but learning from his retreat of his son's incompetent and vicious reign, he emerged, captured and blinded Vladimir, and proclaimed his youngest son Simeon (q.v.) tsar (893). He then returned to his monastery, where he died in the odour of sanctity on May 2, 903. He is reckoned a saint by the Orthodox Church.
Although his campaigns against Serbia (854) and Croatia were unsuccessful, Boris was a strong and influential sovereign, but he is chiefly famous for his conversion to Christianity and his furtherance of the spiritual life of Bulgaria.