Year 240 (CCXL) was a (leap year starting on Wednesday) of the Julian calendar.
|Centuries:||2nd century – 3rd century – 4th century|
|Decades:||210s 220s 230s – 240s – 250s 260s 270s|
|Years:||237 238 239 – 240 – 241 242 243|
|Ab urbe condita||993|
|Balinese saka calendar||161–162|
|Chinese calendar||己未年 (Earth Goat)|
2936 or 2876
— to —
庚申年 (Metal Monkey)
2937 or 2877
|Coptic calendar||−44 – −43|
|- Vikram Samvat||296–297|
|- Shaka Samvat||161–162|
|- Kali Yuga||3340–3341|
|Iranian calendar||382 BP – 381 BP|
|Islamic calendar||394 BH – 393 BH|
|Minguo calendar||1672 before ROC|
|Seleucid era||551/552 AG|
|Thai solar calendar||782–783|
366 or −15 or −787
— to —
367 or −14 or −786
- The Roman Empire is threatened on several fronts at the same time. Africa revolts and tribes in northwest Germania, under the name of the Franks, are raiding the Rhine frontier.
- Maharaja Sri-Gupta becomes Emperor of Gupta.
- Ardashir I, Sassanid king of Persia, destroys Hatra.
- Shapur I becomes co-emperor with his father Ardashir I
- The Kushan Empire falls.
- At the court of Ardashir I, Mani, a young mystic of Ctesiphon, calls himself a prophet and preaches his doctrine, Manichaeism, throughout the Persian Empire.
- Lactantius, Christian writer (d. 320) (approximate date)
- Sporus of Nicaea, Greek mathematician and astronomer (approximate date)
- Zenobia, queen of the Palmyrene Empire (d. 274)
- Ammonius Saccas, Neoplatonic philosopher (approximate date)
- Huang Quan, general of the Kingdom of Shu
- King Midang, king of the Qiang tribe
- Zhou Fang, general of the Kingdom of Wu