metropolitan (archbishop) of Thessalonica (c. 1175–94), humanist scholar, author, and Greek Orthodox reformer whose chronicles, oratory, and pedagogy show him to be one of medieval Byzantium's foremost men of learning.

Before his appointment as a deacon of Constantinople's basilica of Hagia Sophia (Holy Wisdom) and teacher of rhetoric in the Patriarchal school, Eustathius was apparently a monk in the cloister of St. Florus in Constantinople. He held the post of master of petitions in the imperial court and wrote on the literary classics of Greek antiquity, most notably on the works of Homer. Appointed a bishop in 1175, Eustathius had not yet been installed when he was elevated to the post of metropolitan of Thessalonica, a position he held for the remainder of his life.

During the siege and sack of Thessalonica in 1185 by the Normans (Norman) under William II of Sicily, Eustathius bargained with the invaders for the safety of his people. He recounted these events in his De Thessalonica urbe a Normannis capta (“On the Conquest of Thessalonica by the Normans”). Opposing the formalism petrifying the Eastern Church, he criticized clerical complacency in his treatise “On Hypocrisy” and urged the moral and cultural reawakening of monasticism in his famous tract Inquiry into the Monastic Life. Noted for his promotion of sound principles of education and for the preservation of books as well as for his moral example, Eustathius is popularly regarded as a saint by the Greek Orthodox.