|Centuries:||3rd century – 4th century – 5th century|
|Decades:||370s 380s 390s – 400s – 410s 420s 430s|
|Years:||397 398 399 – 400 – 401 402 403|
|400 by topic|
|State leaders – Sovereign states|
|Birth and death categories|
|Births – Deaths|
|Establishment and disestablishment categories|
|Establishments – Disestablishments|
|Ab urbe condita||1153|
|Bahá'í calendar||−1444 – −1443|
|English Regnal year||N/A|
|Chinese calendar||己亥年 (Earth Pig)
3096 or 3036
— to —
庚子年 (Metal Rat)
3097 or 3037
|- Vikram Samvat||456–457|
|- Shaka Samvat||322–323|
|- Kali Yuga||3501–3502|
|Igbo calendar||−600 – −599|
|Iranian calendar||222 BP – 221 BP|
|Islamic calendar||229 BH – 228 BH|
|Minguo calendar||1512 before ROC
|Thai solar calendar||943|
|Wikimedia Commons has media related to 400.|
Year 400 (CD) was a leap year starting on Sunday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Stilicho and Aurelianus (or, less frequently, year 1153 Ab urbe condita). The denomination 400 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 9 – Emperor Arcadius gives his wife Aelia Eudoxia officially the title of Augusta. She is able to wear the purple paludamentum and is depicted in Roman currency.
- Anthemius, praetorian prefect of the East, is sent on an embassy to the Persian capital, Ctesiphon, to congratulate king Yazdegerd I on his accession the year before.
- A riot breaks out in Constantinople, the Great Palace is burned to the ground. Gainas, an ambitious Gothic leader, attempts to evacuate his soldiers out of the city but 7,000 armed Goths are trapped and killed by order of Arcadius. After the massacre, Gainas tries to escape across the Hellespont, but his rag-tag ad hoc fleet is destroyed by Fravitta, a Gothic chieftain in imperial service.
- Winter – Gainas lead the remaining Goths back to their homeland across the Danube. They meet the Huns and are defeated, the Hunnic chieftain Uldin send the head of Gainas to Constantinople where Arcadius received it as a diplomatic gift.
- The Franks establish themselves in the North of the Netherlands.
- The Paeonians (Illyricum) lose their identity (approximate date).
- The Vandals start their westward trek from Dacia and Hungary.
- Construction at Great Zimbabwe begins (approximate date).
- Chrysanthemums are introduced to Japan (approximate date).
- Richu, the eldest son of Nintoku, becomes the 17th emperor of Japan.
- Resurrection and "Two Marys with Angel near the Empty Tomb", panel of a diptych, found in Rome, is made. It is now kept at Castello Sforzesco, Milan (approximate date).
- The Vergilius Vaticanus, an illuminated manuscript, containing fragments of Virgil's Aeneid and Georgics is made in Rome.Yoga Sutras of Patanjali was composed.
- Caelius Aurelianus, Roman physician, is practising his work "De morbis acutis et chronicis" (Concerning Acute and Chronic Illness), a guide to acute and chronic diseases.
- Hypatia, Greek philosoper, distinguishes herself as one of the first women scientists, and becoming head of the Neo-Platonist school at Alexandria.
- The mausoleum of Galerius in Salonica (Greece) is converted into a church.
- Bishops from Gaza (Palestine) arrive at Constantinople to ask Arcadius that he close the pagan temple at Marneum.
- Aspar, Alan patrician and general (magister militum) (approximate date)
- Hydatius, bishop of Aquae Flaviae (modern Chaves, Portugal) (approximate date)
- Salvian, Christian writer (approximate date)
- Castor of Karden, Christian priest and hermit
- Duan, Chinese empress and wife of Murong Bao
- Gainas, Gothic chieftain and general (magister militum)
- Li Lingrong, empress and mother of Jin Xiaowudi
- Lü Guang, emperor of the Di state Later Liang (b. 337)
- Lü Shao, "Heavenly Prince" (Tian Wang) of Later Liang
- Oribasius, Greek medical writer and physician