840
| Years: 836 837 838 839 - 840 - 841 842 843 844 | |
| Decades: 810s 820s 830s - 840s - 850s 860s 870s | |
| Centuries: 8th century - 9th century - 10th century | |
Events
- After the death of Louis the Pious, his sons Lothair, Charles the Bald and Louis the German fight over the division of the empire, with Lothair succeding as Emperor.
- Tang Wu Zong succeeds Tang Wen Zong as emperor of China.
- Foundation of Dublin by Vikings.
- Aed I of Scotland (approximate date)
- Ethelred of Wessex (approximate date)
- Edmund of East Anglia (approximate date)
- June 20: Louis the Pious, king of the Franks, Emperor
- Emperor Junna of Japan