The Christianization of the Rus' people is supposed to have begun in the 860s and was the first stage in the process of Christianization of the East Slavs which continued well into the 11th century. Despite its historical and cultural significance, records detailing the event are hard to come by, and it seems to have been forgotten by the time of Vladimir's Baptism of Kiev in the 980s.

The most authoritative source on the first Christianization of the Rus' is an encyclical letter of Patriarch Photius of Constantinople, datable to early 867. Referencing the Rus'-Byzantine War of 860, Photius informs the Oriental patriarchs and bishops that, after the Bulgars turned to Christ in 863, the Rus' followed suit so zealously that he found it prudent to send to their land a bishop.