John VIII Xiphilinos (Greek: Ιωάννης Η΄ Ξιφιλίνος) (died 2 August 1075), a native of Trebizond, was patriarch of Constantinople from 1064–1075. He was the uncle of John Xiphilinos the Epimator. One of the leading Byzantine intellectuals of his day, a leader of law studies, he was exiled, became a monk, then was made patriarch. He was admired for his legal learning, wisdom, and high character.John VIII also wrote a hagiography of Saint Eugenios of Trebizond.
John Xiphilinos was born in Trebizond about 1010. He pursued studies in Constantinople and eventually became nomophylax of the law school there (ODB 2:1054). Later he became a monk and was eventually selected by Emperor Constantine X (1059–67) to succeed Constantine Leichoudes. In 1072 John presided over an assembly of metropolitans and archbishops at the oratory of St. Alexios in which the question of the election of bishops to vacant sees was discussed. Michael Keroularios had forbidden metropolitans who were resident in Constantinople from participating in such elections. John, however, recognized that metropolitans sometimes had to remain for a long period in the capital due to ecclesiastical business or illness. The assembly with John’s consent decreed that metropolitans who gave the patriarch advance notification of their intent could again vote while resident in Constantinople. After his death his remains were buried at the monastery of Angourion on 2 August 1075.