|Covid-19|September 29 – Emperor Lothair I dies after a 15-year reign (co-ruling with his father Louis the Pious until 840). He divides the Middle Frankish Kingdom between his three sons in an agreement called the Treaty of Prüm—the eldest, Louis II, receives the northern half of Italy and the title of Holy Roman Emperor. The second, Lothair II, receives Lotharingia (the Low Countries and Upper Burgundy). The youngest, Charles, receives Lower Burgundy and Provence.
Spring – King Æthelwulf of Wessex decides to go on a pilgrimage to Rome, accompanied by his youngest son Alfred (age 6) and a large retinue. He divides the kingdom between his two eldest sons; Æthelbald receives the western part of Wessex, while Æthelberht becomes ruler over Kent, Surrey, Sussex and Essex.
Caliph al-Mutawakkil sends a Abbasid army, led by the Turkic general Bugha al-Kabir, to suppress an uprising of rebellious Armenian nakharars. He subdues the country, and deports many Armenian nobles to the caliphal capital of Samarra.
July 17 – Pope Leo IV dies after a 8-year reign, and is succeeded by Benedict III as the 104th pope of Rome. Anastasius is made anti-pope by Lothair I.
Æthelwulf grants churches in Wessex the right to receive tithes. He gives one-tenth of his lands to the Church.
The Slavic alphabet is created by Saints Cyrill and Methodius.
Abu'l-Hasan Ali ibn al-Furat, Muslim vizier (d. 924)
Gerald of Aurillac, Frankish nobleman (approximate date)
Guaimar I of Salerno, Lombard prince (approximate date)
Han Jian, Chinese warlord (d. 912)
Jing Hao, Chinese painter (d. 915)
July 17 – Leo IV, pope of the Catholic Church (b. 790)
September 29 – Lothair I, Frankish king and emperor (b. 795)
November 20 – Theoktistos, Byzantine chief minister
Ahmad ibn Hanbal, Muslim scholar and theologian (b. 780)
Boso the Elder, count of Turin and Valois
Cyngen ap Cadell, king of Powys (Wales)
Drogo of Metz, illegitimate son of Charlemagne (b. 801)
Elisedd ap Cyngen, king of Powys (Wales)
Gozbald, abbot and bishop of Würzburg
Pepin, count of Vermandois (approximate date)
Sahnun ibn Sa'id, Muslim jurist (or 854)
Sico II, prince of Salerno (Italy)
Year 855 (DCCCLV) was a common year starting on Tuesday (link will display the full calendar) of the Julian calendar.