|Centuries:||3rd century – 4th century – 5th century|
|Decades:||360s 370s 380s – 390s – 400s 410s 420s|
|Years:||388 389 390 – 391 – 392 393 394|
|391 by topic|
|State leaders – Sovereign states|
|Birth and death categories|
|Births – Deaths|
|Establishment and disestablishment categories|
|Establishments – Disestablishments|
|Ab urbe condita||1144|
|Chinese calendar||庚寅年 (Metal Tiger)
3087 or 3027
— to —
辛卯年 (Metal Rabbit)
3088 or 3028
|- Vikram Samvat||447–448|
|- Shaka Samvat||313–314|
|- Kali Yuga||3492–3493|
|Iranian calendar||231 BP – 230 BP|
|Islamic calendar||238 BH – 237 BH|
|Minguo calendar||1521 before ROC
|Seleucid era||702/703 AG|
|Thai solar calendar||933–934|
Year 391 (CCCXCI) was a common year starting on Wednesday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Tatianus and Symmachus (or, less frequently, year 1144 Ab urbe condita). The denomination 391 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.
- Emperor Theodosius I establishes Christianity as the official state religion. In the same year Christians sack the Library of Alexandria and burn all the books. All non-Christian temples in the Roman Empire are closed. The eternal fire in the Temple of Vesta at the Roman Forum is extinguished, the Vestal Virgins are disbanded, and "pagan" schools of philosophy are destroyed.
- Quintus Aurelius Symmachus, urban prefect of Rome, pleads for traditional cult practices. He petitions Theodosius I to re-open the pagan temples, but is opposed by Ambrose.
- A Rouran chief named Heduohan (曷多汗) is defeated and killed in battle against the Toba Northern Wei Dynasty. Surviving Rouran move west towards the Gaoche, led by Heduohan's son and successor, Shelun.
- King Gwanggaeto the Great of Goguryeo (Korea) ascends to the throne.
- Flames destroy the great Library of Alexandria, established in the Mouseion in the fourth century BCE. Among the items lost in the fire are works of science, including parchments by the Greek astronomer Aristarchus of Samos asserting that the Earth orbits the Sun, and dozens of dramatic works by Euripides and Sophocles.
- Patriarch Theophilus destroys all pagan tempels in Alexandria under orders from Theodosius I. Christians go on an iconoclastic rampage, smashing religious symbols or monuments through the city and destroying the Temple of Serapis. The "Order of Monks" known as the Parabalani take charge of patrolling the streets.
- Heduohan, chief of the Rouran tribes (killed in battle against the Northern Wei)
- Justina, Roman empress (approximate date)
- Macarius of Egypt, Christian monk and hermit
- Peter of Sebaste, bishop of Armenia
- Zhai Liao "Heavenly Prince" (Tian Wang), founder of the Dingling state Wei