|Covid-19|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 with his fleet from Abydos to Thrace.
February – Duke Borna of Croatia dies after an 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 he 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.
Tahir ibn Husayn, an Iranian general, is appointed to govern Khurasan, as a reward for supporting the Abbasid caliph al-Ma'mun in the Fourth Fitna. This begins the rule of the Tahirid dynasty over Khurasan, which will last until 873.
Gao Pian, general of the Tang Dynasty (d. 887)
Gisela, daughter of Louis the Pious
Ibn Abi Asim, Muslim Sunni scholar (or 822)
Ordoňo I, king of Asturias (approximate date)
May 2 – Liu Zong, general of the Tang Dynasty
December 18 – Theodulf, bishop of Orléans
Arno, archbishop of Salzburg
Artrí mac Cathail, king of Munster (Ireland)
Benedict of Aniane, Frankish monk
Borna, duke (knez) of Croatia
Coenwulf, king of Mercia
Egbert, bishop of Lindisfarne
Guisclafred, Frankish nobleman (approximate date)
Li Su, general of the Tang Dynasty (b. 773)
Tian Hongzheng, general of the Tang Dynasty (b. 764)
Wei Guanzhi, chancellor of the Tang Dynasty (b. 760)
Zheng Yuqing, chancellor of the Tang Dynasty (b. 746)
Year 821 (DCCCXXI) was a common year starting on Tuesday (link will display the full calendar) of the Julian calendar.