Achillius of Larissa was a 4th-century bishop born in Cappadocia who served as the metropolitan of Larissa in Thessaly, Greece. He is primarily recognized for his participation in the First Council of Nicaea, where he stood as one of the 318 attendees. During this council, he became a prominent defender of orthodox Christian doctrine against Arianism. Following the council, he is credited with establishing numerous churches and removing pagan influences within his region.