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This article is about the year 821. For the number, see 821 (number).
|Centuries:||8th century – 9th century – 10th century|
|Decades:||790s 800s 810s – 820s – 830s 840s 850s|
|Years:||818 819 820 – 821 – 822 823 824|
|821 by topic|
|State leaders – Sovereign states|
|Birth and death categories|
|Births – Deaths|
|Establishment and disestablishment categories|
|Establishments – Disestablishments|
|Ab urbe condita||1574|
|Chinese calendar||庚子年 (Metal Rat)
3517 or 3457
— to —
辛丑年 (Metal Ox)
3518 or 3458
|- Vikram Samvat||877–878|
|- Shaka Samvat||743–744|
|- Kali Yuga||3922–3923|
|Japanese calendar||Kōnin 12
|Minguo calendar||1091 before ROC
|Seleucid era||1132/1133 AG|
|Thai solar calendar||1363–1364|
- Thomas the Slav, Byzantine general (tourmarchos), leads a revolt and secures control over most of the Byzantine themes (provinces) in Anatolia. He gets recognition from the Abbasid Caliphate, and concludes a peace treaty with caliph Al-Ma'mun. Emperor Michael II is blockaded in Constantinople, Thomas assembles troops, gathers supplies, and builds siege machines but his first attack on the capital fails. He crosses his fleet from Abydos to Thrace.
- February – Duke Borna of Croatia dies after a 11-year reign as vassal of the Frankish Empire. He is succeeded by his nephew, Vladislav. Emperor Louis I recognizes him as prince of Dalmatia and Liburnia at the Council of Aachen.
- October – Lothair I, co-emperor and eldest son of Louis I, marries Ermengarde in Thionville (north-eastern France). She is the daughter of count Hugh of Tours.
- King Coenwulf of Mercia dies in Basingwerk near Holywell (Wales), while preparing for another assault on Powys, and is buried in Winchcombe Abbey. He is briefly succeeded by his son Cynehelm, but is killed, probably fighting the Welsh, though supposedly through the treachery of his sister Cwenthryth. The Mercian throne passes to Coenwulf's brother, Ceolwulf I.
- Arno, archbishop of Salzburg
- Benedict of Aniane, Frankish monk
- Borna, duke (knez) of Croatia
- Coenwulf, king of Mercia
- Egbert, bishop of Lindisfarne
- Li Su, general of the Tang Dynasty (b. 773)
- December 18 – Theodulf, bishop of Orléans
- Zheng Yuqing, chancellor of the Tang Dynasty (b. 746)
- Mladjov, Ian. "Croatian Rulers" (PDF). Retrieved 2011-05-21.