Year 373 (CCCLXXIII) was a common year starting on Tuesday of the Julian calendar. At the time, it was known as the Year of the Consulship of Augustus and Valens (or, less frequently, year 1126 Ab urbe condita). The denomination 373 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 Valens is converted to Arianism, and orders the persecution of Trinitarian Christians in the Roman East.
• Quintus Aurelius Symmachus becomes proconsul of Africa, and is made a member of the pontifical college.
• Count Theodosius is appointed commander of an expedition to suppress the rebellion of Firmus in Mauretania.
• Valens Aqueduct is inaugurated near Constantinople (modern Istanbul); the aqueduct has a length of 971 meters.

• Battle of the Tanais River: The Huns defeat the Alans near the Don, sending the remnants fleeing westward.

• King Shapur II declares war as a result of Valens' support of Armenia. Emperor Valens makes Antioch his military base for the campaign against Persia.

• Saint Martin of Tours undertakes the Christianization of Gaul.

• Murong Hui, Chinese general and prince of the Later Yan Dynasty (d. 397)
• Murong Sheng (or Daoyun), Chinese emperor of the Later Yan Dynasty (d. 401)
• Synesius of Cyrene, Christian bishop (approximate date)

• May 2 – Athanasius of Alexandria, Egyptian bishop and saint (b. 296)^[1]
• June 9 – Ephrem the Syrian, Syrian Orthodox priest and saint (b. 306)
• Huan Wen (or Yuanzi), Chinese general and regent (b. 312)
• Nerses I (the Great), Armenian catholicos (or patriarch)
• Sun Sheng, Chinese historian and politician (b. 302)

• 
  Saint Ephrem the Syrian
• 
  Saint Athanasius of Alexandria
• 
  Saint Nerses I