Johannes Hymonides, known as John the Deacon of Rome (d. before 882), was towards the middle of the 9th century a monk of Monte Cassino near Rome, and later a deacon of the Roman Church. Possessed of considerable learning, he was closely associated with Anastasius, Librarian of the Roman Church (died 879).
At the instance of Pope John VIII (872-82), John wrote a life of St. Gregory the Great, making use of the works of this pope and of extracts made at an earlier date from the pope's letters in the archives of the Roman Church. The work is divided into four books: in the first he gives an account of the life of Gregory up to the time of his pontificate; in the second, of his activities as pope; in the third, of his teachings; and in the fourth, of his progress in perfection. The life was edited by Jean Mabillon.
He also intended to write a detailed history of the Church, and at his request the aforesaid Anastasius compiled a history in three parts (tripartita) from Greek sources for the use of John, whose purpose, however, was never executed. On the invitation of Bishop Gaudericus of Velletri (867-79), he undertook to re-edit the Gesta Clementis, a life of Pope Clement I (died about end of the 1st century), but did not live to finish the work, which Gaudericus undertook to complete, though it never appeared in full. It is said that in the Bibliothèque Nationale at Paris there is yet in manuscript a short commentary on the Heptateuch based on patristic sources and written by this John.
A letter from a certain Johannes Diaconus to Senarius, "vir illustris", treats of the ceremonies of baptism; it is not however, the work of the John treated here, but of an older deacon of this name.