Prester John, the name given by Europeans in the middle ages to a supposed Christian sovereign or dynasty of sovereigns established in the interior of Asia. The name occurs first in the 11th century, and according to one account he was an eastern potentate dwelling beyond Persia, who, having been converted to Christianity by the spirit of a departed saint, caused his subjects to adopt the same faith. He was at once sovereign and priest of his people (whence his name Prester or Presbyter), and his sway, in its pastoral simplicity and benignity, was compared to that of kings in the patriarchal times. This story was found to be a mere fancy. The belief in the existence of Prester John, however, took a more tangible shape in the 13th century, and on the authority of some Nestorian priests he was said to be identical with Ung Khan, a powerful Tartar chief living in Karakorum, in eastern Tartary, who was overthrown and slain by Genghis Khan. Giovanni Carpini, a Franciscan friar, who in 1246 was sent on a mission to Batu Khan, the grandson of Genghis, failed to discover the Christian monarch or his subjects, but supposed him to be established further to the eastward.

A few years later another Franciscan, Rubruquis, penetrated as far as the court of Batu Khan in central Tartary, and was thence forwarded to Karakorum, the residence of Mangu Khan, and the supposed seat of Prester John. His search for the latter was unavailing, but from a few Nes-torian priests whom he met there, he ascertained that Ung Khan had encouraged the propagation of Christianity in his dominions. The existence of Prester John nevertheless continued to be believed, and as late as the close of the 15th century the Portuguese, who had Beached India by the way of the cape of Good Hope, made fruitless inquiries for him there. About the same time Peres da Covil-ham, a Portuguese, made a journey to Abyssinia in quest of the kingdom of Prester John; and finding the negus or king of Habesh to be a Christian prince, he conferred the title upon him. Mosheim, in his " Ecclesiastical Institutes," conjectures that Prester John may have been a Nestorian priest, who, gaining possession of a throne in eastern Tartary, .transmitted his title as well as his name to his successors.

Others have supposed him to be identical with the grand lama.