Euphrosyne (sometimes spelled Efrasinnia) of Polatsk (or Polacak, Polotsk) (Belarusian: Эўфрасі́ньня По́лацкая) was the granddaughter of a prince of Polacak, Usiaslau.

Euphrosyne was born in 1110. She refused all proposals of marriage and, without her parents' knowledge, ran away to the convent of which her aunt was the abbess and became a nun. Later she founded her own convent. She spent her time copying books, and the money she thus earned she distributed amongst the poor. She also built two churches, and one, the church of The Holy Saviour, still stands today and is considered to be the most precious monument of early Belarusian architecture. Towards the end of her life, she undertook a pilgrimage to the Holy Land, were she died sometime after 1167. Her body, after the conquest of Jerusalem by Saladin in 1187, was carried by the monks to Kiev and deposited there in the Monastery of the Caves. It was only in 1910 that the relics of the saint were brought back to her native town of Polacak.

Euphrosyne is the only East Slav virgin saint. The cross of Saint Efrasinnia was a splendid gem-studded cross created at Efrasinnia's behest by a local master, Lasar Bohsa (Лазар Богша). The famous six-armed golden cross was decorated with enamels and precious stones and presented by her to the church of the Holy Saviour in 1161. Of exquisite beauty , the relic survived centuries of turbulence until World War II, when it mysteriously disappeared during the evacuation of the museum in 1941. Most probably it was stolen by a Soviet soldier, or possibly by a German. In its attempts to trace the whereabouts of this treasure, the government of the Republic of Belarus has looked virtually everywhere, examining even private collections in the United States.