Year 921 (CMXXI) was a common year starting on Monday (link will display the full calendar) of the Julian calendar.
- Summer – King Henry I (the Fowler) defeats his rival Arnulf I (the Bad), duke of Bavaria, in two campaigns. Arnulf is besieged at Regensburg and forced to accept peace negotiations, recognising Henry as sole sovereign of the East Frankish Kingdom (Germany).
- Landulf I, prince of Benevento, supports an anti-Greek Apulian rebellion, ravaging several Byzantine strongpoints as far as Ascoli. The Apulian nobility, professing loyalty to the Byzantine Empire, appoints Landulf as stratego of the Theme of Longobardia.
- September 15 – Ludmila, Bohemian duchess and widow of Bořivoj I, is murdered by her daughter-in-law Drahomíra at Tetín (modern Czech Republic). Ludmila will be canonised and become the patron saint of the Orthodox and the Catholic Church.
- November 7 – Treaty of Bonn: King Charles III (the Simple) and Henry I sign a peace treaty or 'pact of friendship' (amicitia) at a ceremony aboard a ship in the middle of the Rhine, recognising the border between their two Frankish kingdoms.
- A Hungarian mercenary force led by Dursac and Bogát defeats an army of insurgents, who plans to overthrow their ally, Emperor Berengar I, at Brescia. He appoints Giselbert I as count palatine of Bergamo (Northern Italy).
- February 13 – Vratislaus I, duke of Bohemia
- September 15 – Ludmila, Bohemian duchess
- Alexios Mosele, Byzantine admiral
- Elvira Menéndez, queen of Galicia and León
- Harusindan, ruler of the Gilites (Iran)
- Lili ibn al-Nu'man, ruler of the Gilites
- Liu Xun, general of Later Liang (b. 858)
- Ragnall ua Ímair, Viking king of Northumbria
- Richard, duke of Burgundy (b. 858)
- Wang Rong, Chinese warlord (b. 877)
- ^ Timothy Reuter (1999). The New Cambridge Medieval History: Volume III, p. 241. ISBN 978-0-521-36447-8.
- ^ Knight, Judson. Ahmad ibn Fadlan: An Arab Among the Vikings of Russia. Science and Its Times. Ed. Neil Schlager and Josh Lauer. Vol. 2: 700 to 1449. Detroit: Gale, 2001, pp. 32–34. Gale Virtual Reference Library.
- ^ Gilbert Meynier (2010). L'Algérie cœur du Maghreb classique. De l'ouverture islamo-arabe au repli (658-1518). Paris: La Découverte; p. 28.
- ^ Gilbert Meynier (2010). L'Algérie cœur du Maghreb classique. De l'ouverture islamo-arabe au repli (658-1518). Paris: La Découverte; p. 38.