| Honorific Prefix: | Saint |
| Eustathius | |
| Birth Date: | 10th century |
| Death Date: | December 1025 |
| Feast Day: | May 31 |
| Venerated In: | Eastern Orthodox Church |
| Titles: | Patriarch of Constantinople |
| Honorific Prefix: | Saint |
| Eustathius | |
| Patriarch Of: | Ecumenical Patriarch of Constantinople |
| Enthroned: | July 1019 |
| Ended: | December 1025 |
| Religion: | Chalcedonian Christianity |
| Predecessor: | St. Sergius II |
| Successor: | Alexius |
Eustathius (Greek, Modern (1453-);: Εὐστάθιος; died December 1025) was Ecumenical Patriarch of Constantinople from July 1019 to December 1025.
Eustathius was the protopresbyter of the imperial palace when he was raised to the Patriarchal throne by the Emperor Basil II, after the death of Sergius II. Eustathius participated in the efforts of the Byzantines in 1024 to come to an accommodation with the Latin Papacy concerning the widening gap between the Western and Eastern churches, which culminated in the Schism of 1054. At the time of Eustathius, the Papacy claimed dominion over the Christian world, not just primacy, a position which offended Constantinople, the effective spiritual guides of much of the East to include the Russians, Bulgarians and Serbs. Eustathius offered a compromise to Pope John XIX, suggesting that the Orthodox Patriarch would be ecumenic in its own sphere (in suo orbe) in the East as the Papacy was in the world (in universo).[1] It is assumed this was Eustathius' effort to retain control over the Southern Italian churches.[2] While the offer was rejected, there was an acceptance by John of the practice of the Byzantine Rite in the south of Italy in exchange for the establishment of Latin Rite churches in Constantinople.[3]
His successor was Alexius of Constantinople.