Acacius was a fifth-century bishop of Melitene, the capital of the Roman province of Second Armenia. Born into a pious Armenian family that had been childless, he was, by tradition, dedicated to the service of the Church from childhood and entrusted to Bishop Otreius of Melitene, who raised him and advanced him through the orders of reader, deacon, and priest before he himself succeeded to the same see.
He is best remembered as a participant at the Third Ecumenical Council, held at Ephesus in 431, where he stood among the bishops who upheld the teaching of Saint Cyril of Alexandria against Nestorius, defending the union of the divine and human natures in Christ and the title of the Virgin Mary as Theotokos. He reposed in peace around the year 435.