The 840s decade ran from January 1, 840, to December 31, 849.
Significant people
- Al-Mu'tasim
- Al-Wathiq
- Alfred the Great
- Louis the Pious
- Charles the Bald
- Ermentrude of Orléans
- Louis the Stammerer
- Louis the German
- Lothair I
- Kenneth I of Scotland
- Ragnar Lodbrok
- Michael III