|Centuries:||3rd century – 4th century – 5th century|
|Decades:||330s 340s 350s – 360s – 370s 380s 390s|
|Years:||363 364 365 – 366 – 367 368 369|
|366 by topic|
|State leaders – Sovereign states|
|Birth and death categories|
|Births – Deaths|
|Establishment and disestablishment categories|
|Establishments – Disestablishments|
|Ab urbe condita||1119|
|Chinese calendar||乙丑年 (Wood Ox)
3062 or 3002
— to —
丙寅年 (Fire Tiger)
3063 or 3003
|- Vikram Samvat||422–423|
|- Shaka Samvat||288–289|
|- Kali Yuga||3467–3468|
|Iranian calendar||256 BP – 255 BP|
|Islamic calendar||264 BH – 263 BH|
|Minguo calendar||1546 before ROC
|Seleucid era||677/678 AG|
|Thai solar calendar||908–909|
Year 366 (CCCLXVI) was a common year starting on Sunday of the Julian calendar. At the time, it was known as the Year of the Consulship of Gratianus and Dagalaifus (or, less frequently, year 1119 Ab urbe condita). The denomination 366 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.
- January 2 – The Alamanni cross the frozen Rhine in large numbers, and invade the Gallic provinces. They capture Alsace and a large part of the Swiss Plateau.
- April – Emperor Valens defeats the troops of Procopius in the Battle of Thyatira, bringing an end to his revolt; Serenianus and Marcellus are killed. He flees the battlefield, but is executed by Valens.
- Valens builds a pontoon bridge across the Danube and drives the Visigoths farther north, where they will come under pressure from the advancing Huns.
- Winter – Emperor Valentinian I appoints Jovinus, his Master of the Horse (Magister Equitum), general of the army. He defeats the Alamanni in three successive battles and pushes them out of Gaul.
Arts and sciences
- The Tabula Peutingeriana, a map showing Roman possessions and roads, is created about this time.
- January 31 – Athanasius of Alexandria returns from his fifth exile. He spent four months in his ancestral tomb outside Alexandria.
- Buddhist monk Lè Zūn has a vision of "golden rays of light shining down on 1,000 Buddha's", resulting in the creation of the Mogao Caves.
- October 1 – Pope Liberius dies after a 14-year reign and is succeeded by Damasus I as 37th pope. Romans unhappy with this choice elect the antipope Ursicinus.
- May 27 – Procopius, Roman usurper
- September 24 – Pope Liberius
- Acacius of Caesarea, bishop and Christian leader of Arianism
- Marcellus, general and Roman usurper
- Serenianus, general of the Imperial Guard
- Yu Daolian, empress and wife of Jin Feidi
- Zhi Dun, Chinese Buddhist monk and philosopher (b. 314)