840 deaths

King of the Franks

thumb|Einhard as scribe. Manuscript depiction from 1050

Emperor of Japan

emperor of the Tang Dynasty

Agobard of Lyon (840) was a Spanish-born priest and archbishop of Lyon, during the Carolingian Renaissance. The author of multiple treatises, ranging in subject matter from the iconoclast controversy to Spanish Adoptionism to critiques of the Carolingian royal family, Agobard is best known for his critiques of Jewish religious practices and political power in the Frankish-Carolingian realm. He was succeeded by Amulo of Lyons.

Wigstan (, ; died c. 840 AD), also known as Saint Wystan, was the son of Wigmund of Mercia and Ælfflæd, daughter of King Ceolwulf I of Mercia.

Saint Ansovinus () (d. 816) was a bishop of Camerino, and is the patron saint of agriculture. His feast day is 13 March.

Duke of Naples

Ninth Ismaili Imam (790–840)

Arab Abbasid military commander

Czimislav () was a 9th-century King of the Sorbs. The Saxons won a battle at Kesigesburg and Czimislav was killed in 840. He was part of the Colodici, a Sorbian sub-tribe.

khagan of Uyghurs

Al-Jarmī, full name Abū ‘Umar Ṣāliḥ ibn Isḥāq al-Bajīli al-Jarmī () (d.840 AD/ 225 AH), was an influential grammarian of the Basra school during the Islamic Golden Age, who took part in learned discussions at Baghdād.