Didymus (?309-?394), surnamed the Blind, was an ecclesiastical writer of Alexandria, was born about the year 309.

Although he became blind at the age of four, before he had learned to read; he succeeded in mastering the whole circle of the sciences then known; and on entering the service of the Church he was placed at the head of the Catechetical school in Alexandria, where he lived and worked till almost the close of the century. Among his pupils were Jerome and Rufinus.

He was a loyal follower of Origen, though stoutly opposed to Arian and Macedonian teaching. Such of his writings as survive show a remarkable knowledge of scripture, and have distinct value as theological literature. Among them are the De Trinitate, De Spiritu Sancto (Jerome's Latin translation), Adversus Manichaeos, and notes and expositions of various books, especially the Psalms and the Catholic Epistles.