4. Bishop of Constantinople, succeeded Gennadius A. D. 471, after being at the head of the Orphan Asylum of that city.
He distinguished himself by defending the Council of Chalcedon against the emperor Basiliscus, who favoured the Monophysite heresy. Through his exertions Zeno, from whom Basiliscus had usurped the empire, was restored (A. D. 477), but the Monophysites meanwhile had gained so much strength that it was deemed advisable to issue a formula, conciliatory from its indefiniteness, called the Henoticon, A. D. 482. Acacius was led into other concessions, which drew upon him, on the accusation of John Talaia, against whom he supported the claims of Peter Mongus to the See of Alexandria, the anathema of Pope Felix II. A. D. 484. Peter Mongus had gained Acacius's support by professing assent to the canons of Chalcedon, though at heart a Monophysite. Acacius refused to give up Peter Mongus, but retained his see till his death, A. D. 488.
There remain two letters of his, one to Pope Simplicius, in Latin (see Conciliorum Nova Collection à Mansi,
vol. vii. p. 982), the other to Peter Fullo, Archbishop of Antioch, in the original Greek. (Ibid.