|Covid-19|March – Robert the Strong is appointed margrave of Neustria by King Charles the Bald. He re-establishes the Breton March, and extends his remit by campaigning against Salomon, duke 'king' of Brittany. Robert hires a combined Seine-Loire fleet for 6,000 pounds of silver, 'before Salomon can ally with them against him'. In return, Salomon enlists 12 Viking ships under the command of Hastein, to raid the county of Maine, which, with Anjou, becomes squeezed between Brittany and Neustria.
Carloman, eldest son of King Louis the German, revolts against his father. He is captured, but manages to escape to the Ostmark (or 862).
Summer – Viking raiders sack the cities of Paris, Cologne, Aachen, Worms and Toulouse.
December 11 – Caliph al-Mutawakkil is murdered by his Turkish guard, starting the period of troubles known as the "Anarchy at Samarra" (861–870). He is succeeded by his son Al-Muntasir, as ruler of the Abbasid Caliphate.
Ya'qub ibn al-Layth, a Muslim military leader, founds the Saffarid Dynasty. He rules over parts of Khurasan and eastern Iran, and establishes his capital at Zaranj (modern Afghanistan).
Al-Mutawakkil orders the construction of a Nilometer on Rhoda Island in central Cairo, supervised by the Persian astronomer Ahmad ibn Muhammad ibn Kathir al-Farghani.
Abdullah ibn al-Mu'tazz, Muslim poet (d. 908)
Abu Bakr Shibli, Muslim sufi (d. 946)
Al-Mu'tadid, Muslim caliph (or 854)
Heongang, king of Silla (approximate date)
April 6 – Prudentius, bishop of Troyes
al-Mutawakkil, Abbasid caliph (b. 822)
al-Fath ibn Khaqan, chief confidante and councillor of al-Mutawakkil
Ahmad ibn Muhammad ibn Kathir al-Farghani, Persian astronomer
Álvaro of Córdoba, Mozarab scholar and theologian
Ansovinus, bishop of Camerino (approximate date)
Bai Minzhong, chancellor of the Tang Dynasty (b. 792)
Gladilanus, Galician clergyman (approximate date)
Gregory of Khandzta, Georgian archimandrite (b. 759)
Heonan, king of Silla (Korea)
Lando I, count of Capua
Meinrad of Einsiedeln, German hermit and martyr
Pribina, Slavic prince (approximate date)
Samuel of Kakheti, Georgian prince
Year 861 (DCCCLXI) was a common year starting on Wednesday (link will display the full calendar) of the Julian calendar.