Boris I, also known as Boris-Mikhail (Michael) and Bogoris (died 2 May 907), is the Knyaz (Prince) of the First Bulgarian Empire in 852–889.

At the time of his baptism in 864, Boris is named Michael after his godfather, Emperor Michael III.

The historian Steven Runciman called him one of the greatest persons in history.

Despite a number of military setbacks, the reign of Boris I is marked with significant events that shape Bulgarian and Slavic history.

With the Christianization of Bulgaria, the traditional state religion is abolished.

A skillful diplomat, Boris I successfully exploits the conflict between the Patriarchate of Constantinople and the Papacy to secure an autocephalous Bulgarian Church, thus dealing with the nobility's concerns about Byzantine interference in Bulgaria's internal affairs.

When in 885 the disciples of Saints Cyril and Methodius are banished from Great Moravia, Boris I gives them refuge and provides assistance to develop the Slavic alphabet and literature.

After he abdicates in 889, his eldest son and successor tries to restore the old pagan religion but is deposed by Boris I.

During the Council of Preslav which follows that event, the Byzantine clergy is replaced with Bulgarians, and the Greek language is replaced with Old Church Slavonic as an official language of the Church and the state.

He is regarded as a saint in the Orthodox Church, as the Prince and baptizer of Bulgaria, and as Equal-to-the-Apostles, with his feast day observed on May 2.