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 – Battle of Thyatira: Emperor Valens defeats the troops of Procopius, bringing an end to his revolt; Serenianus and Marcellus are killed. Procopius 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 has 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 Buddhas", 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 16 – Ajabel, Roman Catholic martyr
- May 27 – Procopius, Roman usurper
- August 1 – Leo of Montefeltro, Roman Catholic bishop and saint
- September 3 – Saint Marinus
- September 24 – Pope Liberius