|856 by topic|
|Ab urbe condita||1609|
|Balinese saka calendar||777–778|
|Chinese calendar||乙亥年 (Wood Pig)
3552 or 3492
— to —
丙子年 (Fire Rat)
3553 or 3493
|- Vikram Samvat||912–913|
|- Shaka Samvat||777–778|
|- Kali Yuga||3956–3957|
|Japanese calendar||Saikō 3
|Minguo calendar||1056 before ROC
|Seleucid era||1167/1168 AG|
|Thai solar calendar||1398–1399|
982 or 601 or −171
— to —
983 or 602 or −170
|Wikimedia Commons has media related to 856.|
- 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 (approximate date) – An earthquake in Corinth in Greece kills an estimated 45,000 people.
- December 3 – Another earthquake strikes the Abbasid Caliphate (modern-day Tunisia), also killing an estimated 45,000 people.
- December 22 – Another earthquake strikes Damghan (modern-day Iran), killing an estimated 200,000 people.
- October 24 – 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 6 – Fujiwara no Nagara, Japanese statesman (b. 802)
- August 16 – Theutbald I, bishop of Langres
- Ali ibn Ismail, Muslim 7th Imam (b. 752)
- Florinus of Remüs, Frankish priest and martyr
- 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