366 (CCCLXVI) was a common year starting on Sunday of the Julian calendar, the 366th year of the Common Era (CE) and Anno Domini (AD) designations, the 366th year of the 1st millennium, the 66th year of the 4th century, and the 7th year of the 360s decade. As of the start of 366, the Gregorian calendar was 1 day ahead of the Julian calendar, which was the dominant calendar of the time.

• January 31 – Athanasius of Alexandria returns from his fifth exile.
• April – Emperor Valens defeats the troops of Procopius in the Battle of Thyatira.
• Valens builds a pontoon bridge across the Danube. This lets the Visigoths travel farther north.
• The Tabula Peutingeriana, a map showing Roman places and roads, is made about this time.
• October 1 – Damasus I becomes 37th pope.

• Yao Xing, emperor of the Chinese Qiang state Later Qin

• Acacius of Caesarea, bishop and Christian leader of Arianism
• September 24 – Pope Liberius
• Marcellus, general and Roman usurper
• May 27 – Procopius, Roman usurper
• Yu Daolian, empress and wife of Jin Feidi
• Zhi Dun, Chinese Buddhist monk and philosopher