Achillas was the 18th Patriarch of Alexandria, reigning from 312 to 313.
He was born in Alexandria, Egypt, and was renowned for his knowledge and piety; this was why Pope Theonas had ordained him priest and appointed him head of the Catechetical School of Alexandria upon the departure of Pierius. He was apparently very highly thought of for his work in Greek philosophy and theological science, as Pope Athanasius later described him by the honorific "Achillas the Great".