|840 by topic|
|Ab urbe condita||1593|
|Balinese saka calendar||761–762|
|Chinese calendar||己未年 (Earth Goat)
3536 or 3476
— to —
庚申年 (Metal Monkey)
3537 or 3477
|- Vikram Samvat||896–897|
|- Shaka Samvat||761–762|
|- Kali Yuga||3940–3941|
|Japanese calendar||Jōwa 7
|Minguo calendar||1072 before ROC
|Seleucid era||1151/1152 AG|
|Thai solar calendar||1382–1383|
966 or 585 or −187
— to —
967 or 586 or −186
|Wikimedia Commons has media related to 840.|
- June 20 – Emperor Louis the Pious falls ill and dies at his hunting lodge, on an island in the Rhine, near his imperial palace at Ingelheim, while suppressing a revolt. His eldest son Lothair I succeeds him as Holy Roman Emperor, and tries to seize all the territories of the late Charlemagne. The 17-year-old Charles the Bald becomes king of the Franks, and joins the fight with his half-brother Louis the German in resisting Lothair.
- King Wigstan of Mercia, grandson of former ruler Wiglaf (see 839), declines his kingship and prefers the religious life. He asks his widowed mother, Princess Ælfflæd, to act as regent. A nobleman of the line of the late king Beornred, named Berhtric, wishes to marry her but he is a relative. Wigstan refuses the match, and is murdered by followers of Berhtric at Wistow. He is buried at Repton Abbey, and later revered as a saint. The Mercian throne is seized by Berhtric's father, Beorhtwulf.
- Vikings make permanent settlements with their first 'wintering over', located at Lough Neagh in Northern Ireland (approximate date).
- Emperor Wen Zong (Li Ang) dies after a 13-year reign, in which he has failed to break the power of his palace eunuchs. He is succeeded by his brother Wu Zong, as Chinese ruler of the Tang Dynasty.
- The Yenisei Kirghiz settle along the Yenisei River, and sack with a force of around 80,000 horsemen the Uyghur capital, Ordu-Baliq (driving the Uyghurs out of Mongolia). This ends the Uyghur Khaganate.
- January 19 – Michael III, Byzantine emperor (d. 867)
- October 25 – Ya'qub ibn al-Layth al-Saffar, founder of the Saffarid dynasty (d. 879)
- Abu al-Hassan al-Nuri, Muslim Sufi (approximate date)
- Adalhard II, Frankish nobleman (approximate date)
- Berengaudus, Benedictine monk (d. 892)
- Clement of Ohrid, Bulgarian scholar (approximate date)
- Eudokia Ingerina, Byzantine empress (approximate date)
- Hucbald, Frankish music theorist (or 850)
- Lothar I, Frankish nobleman (d. 880)
- Notker the Stammerer, Benedictine monk (approximate date)
- Richardis, Frankish empress (approximate date)
- Sunyer II, Frankish nobleman (approximate date)
- Theodard, archbishop of Narbonne (approximate date)
- Theodore II, pope of the Catholic Church (d. 897)
- Unruoch III, margrave of Friuli (approximate date)
- March 14 – Einhard, Frankish scholar
- June 11 – Junna, emperor of Japan (b. 785)
- June 16 or 839 – Rorgon I, Frankish nobleman
- June 20 – Louis the Pious, king of the Franks (b. 778)
- Agobard, archbishop of Lyon (b. 779)
- Andrew II, duke of Naples
- Ansovinus, archbishop of Camerino
- Czimislav, king of the Sorbs (approximate date)
- He Jintao, general of the Tang Dynasty
- Hilduin, archbishop of Paris (b. 775)
- Li Chengmei, prince of the Tang Dynasty
- Li Rong, prince of the Tang Dynasty
- Muhammad at-Taqi, Muslim ninth Ismā'īlī imam (or 839)
- Salmawaih ibn Bunan, Muslim physician
- Wen Zong, emperor of the Tang Dynasty (b. 809)
- Wigstan, king of Mercia (approximate date)
- Yang, consort and concubine of Wen Zong
- Zaluckyj & Zaluckyj, "Decline", pp. 238–239.
- History of Central Asia.