Gennadius II (lay name Georgios Scholarios) (died circa 1473), patriarch of Constantinople from 1454 to 1464, philosopher and theologian, was one of the last representatives of Byzantine learning, and a strong advocate of Aristotelian philosophy in the Church.

He appears to have been born at Constantinople about 1400 and to have entered the service of the emperor John VIII Palaeologus as a theological advisor. Georgios first appears in history as a presence at the Council of Basel, held in 1438 in Ferrara and Florence with the object of bringing about a union between the Greek and Latin Churches. At the same council appeared the celebrated Platonist, Gemistus Pletho, the most powerful opponent of the then dominant Aristotelianism, and consequently an antagonist of Georgios.

In church matters, as in philosophy, the two were opposed — Pletho maintaining strongly the principles of the Greek Church, and being unwilling to accept union of the Greek and Latin churches through compromise, while Georgios, more cautious, pressed the necessity for union on doctrinal grounds, and was instrumental in drawing up a form which from its vagueness and ambiguity might be accepted by both parties.

Georgios was at a disadvantage because, being a layman, he could not directly take part in the discussions of the council. But on his return to Greece his views changed, apparently at the behest of his mentor Markos Eugenikos, and he became a firm and vocal opponent of the union he had previously urged. After the death of John VIII Palaeologus in 1448, he became a monk at the monastery of Pantokrator and took the name Gennadius.

In 1453, after the Fall of Constantinople, Gennadius was taken prisoner by the Turks. Mehmed II, finding that the patriarchal chair had been vacant for some time, and wanting to use the Church to stabilize his empire, resolved to elect someone to the office, and the sultan compelled Gennadius to accept the title. Mehmed gave Gennadius both ecclesiastical and political authority, and as a result, under Gennadius, the Greek Orthodox Church became a civil as well as a religious entity.

In the spring of 1454 he was consecrated by the metropolitan of Heraclea, but, since both the Church of St. Sophia and the palace of the patriarch were now in the hands of the Ottoman Empire, he took up his residence successively in two monasteries of the city. While holding the episcopal office Gennadius drew up, apparently for the use of Mehmed, a confession or exposition of the Christian faith, which was translated into Turkish by Ahmed, judge of Beroea (and first printed by A. Brassicanus at Vienna in 1530).

Gennadius was unhappy as patriarch, and tried to abdicate his position at least twice. Eventually, he found the tensions between the Greeks and the Ottomans overwhelming, and he retired to the monastery of John the Baptist near Serrae in Macedonia, where he died in about 1473. About 100 of his alleged writings exist, the majority in manuscript and of doubtful authenticity.

As far as is known, his writings may be classified into philosophical (interpretations of Aristotle, Porphyry, and others, translations of Petrus Hispanus and Thomas Aquinas, and defenses of Aristotelianism against the recrudescence of Neoplatonism) and theological and ecclesiastical (partly concerning the union and partly defending Christianity against Muslims, Jews, and pagans), in addition to numerous homilies, hymns, and letters.