|Covid-19|March 15 – Emperor Michael III overthrows the regency of his mother Theodora and the logothete Theoktistos. He appoints his uncle Bardas as the de facto regent and co-ruler of the Byzantine Empire.
King Charles the Bald cedes the county of Maine to Erispoe, ruler (duke) of Brittany—this in return for an alliance against the Vikings.
King Ordoño I of Asturias is said to have begun the repopulation of the town of León in the northwest of Spain (approximate date).
October 1 – King Æthelwulf of Wessex marries the 12- or 13-year-old Judith, daughter of Charles the Bald, at Verberie (Northern France). She is crowned queen and anointed by Hincmar, archbishop of Reims. The marriage is a diplomatic alliance between Wessex and the West Frankish Kingdom.
Winter – Æthelwulf returns to Wessex to face a revolt by his eldest son Æthelbald, who usurps the throne. Æthelwulf agrees to give up the western part of his kingdom, in order to avoid a civil war. He keeps control over Sussex, Surrey, Essex and Kent, over which Prince Æthelberht has presided.
November – An earthquake in Corinth in Greece kills an estimated 45,000 people (approximate date).
December 22 – Another earthquake strikes Damghan (modern Iran), killing an estimated 200,000 people.
Li Keyong, Shatuo governor (jiedushi) (d. 908)
Li Maozhen, Chinese warlord and king (d. 924)
January 7 – Aldric, bishop of Le Mans
February 4 – Rabanus Maurus, archbishop of Mainz
August 16 – Theutbald I, bishop of Langres
Ali ibn Ismail, Muslim 7th Imam (b. 752)
Florinus of Remüs, Frankish priest and martyr
Fujiwara no Nagara, Japanese statesman (b. 802)
Godfrid Haraldsson, Viking chieftain (approximate date)
Guerin, Frankish nobleman (or 845)
Ilyas ibn Asad, Muslim emir (approximate date)
Muhammad I Abu 'l-Abbas, Muslim emir
Year 856 (DCCCLVI) was a leap year starting on Wednesday (link will display the full calendar) of the Julian calendar.