840Centuries: 8th century - 9th century - 10th century
- 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)