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Year 812 (DCCCXII) was a leap year starting on Thursday (link will display the full calendar) of the Julian calendar, the 812th year of the Common Era (CE) and Anno Domini (AD) designations, the 812th year of the 1st millennium, the 12th year of the 9th century, and the 3rd year of the 810s decade.

Events

January 11 – Ex-emperor Staurakios, a son of Nikephoros I, dies of putrefaction in his wounds (see 811) in a monastery. He has reigned only two months and eight days, before being exiled by senior officials in Constantinople.
Emperor Michael I re-opens peace negotiations with the Franks, and recognizes Charlemagne as emperor (basileus) of the Frankish Empire. In exchange for this recognition, Venice is returned to the Byzantine Empire.
Byzantine–Bulgarian War: The Bulgars, led by Krum, ruler (khan) of the Bulgarian Empire, launch an invasion against the Byzantines. They capture the fortress cities of Develt and Mesembria, near the Black Sea.^[1]

Charlemagne conquers Catalonia, as far south as the River Ebro and the Balearic Islands. The counties come under the rule of Bera, count of Barcelona. He signs a three-year peace treaty with the Emirate of Córdoba.
Charlemagne issues the Capitulare de villis, concerning the rights of a feudal landholder and the services owed by his dependents. It also contains the names of some 89 plants, of which most are used medically.
The Republic of Amalfi sends galleys to support the Byzantine general (strategos) of Sicily, Gregorio, against the Aghlabid invaders. It is one of the earliest evidences of the independence of the city.^[2]
At the death of king Hemming of Denmark two claimants to the throne, Sigfred and Anulo, meet in battle but are both killed. Harald and Reginfrid, brothers of Anulo, becomes joint kings of Denmark.

Fourth Fitna: Forces loyal to al-Ma'mun, led by Tahir ibn Husayn, blockade Baghdad, which is loyal to al-Ma'mun's brother, Caliph al-Amin, and begin the year-long Siege of Baghdad.

The Chinese government takes over the issuing of paper bank drafts, the ancestor of paper money.

^ Fine, John V. A. Jr. (1991) [1983]. The Early Medieval Balkans: A Critical Survey from the Sixth to the Late Twelfth Century. Ann Arbor: University of Michigan Press. ISBN 0-472-08149-7.
^ Benvenuti, Gino (1985). Le Repubbliche Marinare. Amalfi, Pisa, Genova e Venezia. Rome: Newton & Compton Editori. p. 13. ISBN 88-8289-529-7.
^ Talbot, Alice-Mary Maffry (1996). "Life of St. Theodora of Thessalonike". Holy Women of Byzantium: Ten Saints' Lives in English Translation. Dumbarton Oaks. pp. 159–160. ISBN 978-0-88402-248-0. Retrieved March 3, 2024.