|Centuries:||4th century – 5th century – 6th century|
|Decades:||370s 380s 390s – 400s – 410s 420s 430s|
|Years:||400 401 402 – 403 – 404 405 406|
|403 by topic|
|State leaders – Sovereign states|
|Birth and death categories|
|Births – Deaths|
|Establishment and disestablishment categories|
|Establishments – Disestablishments|
|Ab urbe condita||1156|
|Chinese calendar||壬寅年 (Water Tiger)
3099 or 3039
— to —
癸卯年 (Water Rabbit)
3100 or 3040
|- Vikram Samvat||459–460|
|- Shaka Samvat||325–326|
|- Kali Yuga||3504–3505|
|Iranian calendar||219 BP – 218 BP|
|Islamic calendar||226 BH – 225 BH|
|Minguo calendar||1509 before ROC
|Seleucid era||714/715 AG|
|Thai solar calendar||945–946|
Year 403 (CDIII) was a common year starting on Thursday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Theodosius and Rumoridus (or, less frequently, year 1156 Ab urbe condita). The denomination 403 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.
- June – Battle of Verona: The Visigoths under command of king Alaric I invade Italy again and advance through the Brenner Pass. Stilicho with an army of 30,000 men, defeats the Goths north of Verona. Alaric makes a truce and withdraws eastward to Illyricum.
- Emperor Honorius and Stilicho are honored with a triumphal march for the victories against the Goths and Vandals. This becomes the last victory celebrated in Rome.
- Theodosius II, age 2, becomes consul of the Eastern Roman Empire.
- Hui Yuan argues that Buddhist monks should be exempt from bowing to the emperor.
- Asin of Baekje allies with Silla against Gwanggaeto the Great of Goguryeo (Korea).
- The Synod of the Oak deposes and banishes John Chrysostom, bishop of Constantinople, but shortly afterward he is recalled only to be banished again.
- Epiphanius of Salamis, Church Father
- Severin, bishop of Cologne
- Sima Daozi, regent of the Eastern Jin Dynasty (b. 364)