Year 412 (CDXII) was a leap year starting on Monday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Honorius and Theodosius (or, less frequently, year 1165 Ab urbe condita). The denomination 412 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
- The Visigoths, led by king Ataulf, move into the south of Gaul. He established his residence at Narbonne and makes an alliance with emperor Honorius against the usurper Jovinus.
- Emperor Jovinus elevates his brother Sebastianus as co-emperor (Augustus) and takes control of Gaul.
- Heraclianus, governor (Comes Africae), revolts against Honorius and proclaimed himself Augustus. He interrupts the grain supply to Rome. Honorius condemns him and his supporters to death with an edict at Ravenna.
- The Theodosian Walls are constructed at Constantinople during the reign of emperor Theodosius II. The work is carried out under supervision of Anthemius, notable praetorian prefect of the East.
- Winter – Olympiodorus, historical writer, is sent on an embassy by Honorius and sails in stormy weather around Greece up the Black Sea to meet the Huns who are located on the middle Danube (modern Bulgaria).
- The forts on the west bank of the Danube, which were destroyed by the Huns, are rebuilt, and a new Danubian fleet is launched.
- An edict of Honorius outlaws Donatism.
- Cyril of Alexandria becomes Patriarch of Alexandria.
- Lazarus, bishop of Aix-en-Provence, and Herod, bishop of Arles, are expelled from their sees on a charge of Manichaeism.
- Fa-Hien, Chinese Buddhist monk, spends 2 years in Ceylon and is more than 200 days at sea as storms drive his ship off its course, but returns with sacred Buddhist texts back to China (see 414).
- Lu Huinan, empress dowager of the Liu Song Dynasty (d. 466)
- February 8 – Proclus, Greek Neoplatonist philosopher (d. 485)