|Covid-19|February 21 – Guy III, duke of Spoleto, is crowned Holy Roman Emperor by Pope Stephen V. His son Lambert is proclaimed king of Italy, at the capital of Pavia in Lombardy.
Summer – Orso, Lombard prince of Benevento, is deposed after the capture of Benevento by the Byzantines. Benevento becomes the capital of the thema of Longobardia.
Battle of Leuven: Viking raiders on the Dyle River (near Leuven), in modern-day Flanders, suffer a crushing defeat by Frankish forces, under King Arnulf of Carinthia.
Muslim forces led by Abdullah ibn Muhammad al-Umawi, Umayyad emir of Córdoba, defeat the rebel leader Umar ibn Hafsun at Poley, in Al-Andalus (modern Spain).
June 2 – Al-Muwaffaq, an Abbasid prince, dies at the capital of Baghdad. His son Al-Mu'tadid is recognized as regent, and second heir of the Abbasid Caliphate.
February 25 – Fujiwara no Mototsune, Japanese statesman, dies. He had forced the resignation of Emperor Yōzei, and became head of the Fujiwara clan.
September 14 – Pope Stephen V dies after a 6-year reign. He is succeeded by Formosus, former cardinal bishop of Portus, as the 111th pope of the Catholic Church.
Abd al-Rahman III, Muslim caliph (or 889)
Gao Conghui, prince and ruler of Jingnan (d. 948)
Lin Ding, Chinese official and chancellor (d. 944)
February 25 – Fujiwara no Mototsune, Japanese regent (b. 836)
June 2 – Al-Muwaffaq, Muslim prince and regent (b. 842)
June 25 – Sunderolt, archbishop of Mainz
September 14 – Stephen V, pope of the Catholic Church
October 23 – Yazaman al-Khadim, Muslim emir
Bernard, illegitimate son of Charles the Fat (or 892)
Chen Yan, Chinese warlord and governor
Enchin, Japanese Buddhist monk (b. 814)
Gu Yanlang, Chinese warlord and governor
Isma'il ibn Bulbul, Muslim official and vizier
Wang Hui, chancellor of the Tang Dynasty
Year 891 (DCCCXCI) was a common year starting on Friday (link will display the full calendar) of the Julian calendar.