Gaius Marius Victorinus (also known as Victorinus Afer; fl. 4th century) was a Roman grammarian, rhetorician and Neoplatonic philosopher. Victorinus was African by birth and experienced the height of his career during the reign of Constantius II. He is also known for translating two of Aristotle's books from ancient Greek into Latin: the Categories and On Interpretation (De Interpretatione). Victorinus had a religious conversion, from being a pagan to a Christian, "at an advanced old age" (c. 355).
Victorinus, at some unknown point, left Africa for Rome (hence some modern scholars have dubbed him Afer), probably for a teaching position, and had great success in his career, eventually being promoted to the lowest level of the senatorial order. That promotion probably came at the time when he received an honorific statue in the Forum of Trajan in 354 (Jerome supplied biographical information but was not his student). Victorinus' religious conversion from Platonism to Christianity (c. 355), "at an advanced old age" according to Jerome, made a great impression on Augustine of Hippo, as recounted in Book 8 of the latter's Confessions. His conversion is historically important in foreshadowing the conversion of more and more of the traditionally pagan intellectual class, from the gods who in pagan belief had made Rome great. Victorinus' conversion, even though criticized by some scholars as purely intellectualist, was undoubtedly sincere, as events connected with the revival of paganism initiated by the last pagan emperor, Julian the Philosopher (dubbed "Julian the Apostate" by Christians) came to show.