|Centuries:||3rd century – 4th century – 5th century|
|Decades:||310s 320s 330s – 340s – 350s 360s 370s|
|Years:||337 338 339 – 340 – 341 342 343|
|340 by topic|
|State leaders – Sovereign states|
|Birth and death categories|
|Births – Deaths|
|Establishment and disestablishment categories|
|Establishments – Disestablishments|
|Ab urbe condita||1093|
|Bahá'í calendar||−1504 – −1503|
|English Regnal year||N/A|
|Chinese calendar||己亥年 (Earth Pig)
3036 or 2976
— to —
庚子年 (Metal Rat)
3037 or 2977
|- Vikram Samvat||396–397|
|- Shaka Samvat||262–263|
|- Kali Yuga||3441–3442|
|Igbo calendar||−660 – −659|
|Iranian calendar||282 BP – 281 BP|
|Islamic calendar||291 BH – 290 BH|
|Minguo calendar||1572 before ROC
|Thai solar calendar||883|
|Wikimedia Commons has media related to 340.|
Year 340 (CCCXL) was a leap year starting on Tuesday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Acindynus and Valerius (or, less frequently, year 1093 Ab urbe condita). The denomination 340 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.
- Constantinople, capital of Emperor Constantius II, becomes the largest city in the world, taking the lead from Rome, capital of his brother Constans I.
- Emperor Constantine II crosses the Alps and attacks the army of Constans I at Aquileia in northern Italy. When he is ambushed and killed by Constans, Constans is left sole ruler of the Western Roman Empire, with his other brother, Constantius II, Emperor of the Eastern Roman Empire.
- Pope Julius I inveighs against Arianism at the Council of Rome.
- Acacius succeeds Eusebius as bishop in the see of Caesarea.
- Wulfila spreads evangelism among the Goths for 7 years.
- Aurelius Ambrose, bishop of Milan (approximate date)
- Amphilochius, bishop of Iconium (approximate date)
- Jerome, priest and Bible translator (approximate date)
- Justina, Roman empress (approximate date)
- Peter of Sebaste, bishop of Armenia (approximate date)
- Quintus Aurelius Symmachus, Roman consul and pagan (approximate date)
- Constantine II, Roman emperor (b. 316)
- Macrina the Elder, saint and mother of Basil the Elder (approximate date)
- Yu Liang, general of the Jin Dynasty (b. 279)