|Covid-19|Byzantine–Arab War: Emperor Theophilos invades the Abbasid dominions, and reaches the Euphrates River in north-eastern Syria. He captures and sacks the city of Tarsus, but is defeated in Cappadocia.
Summer – Muslim Arabs under Caliph Al-Ma'mun launch an invasion into Anatolia (modern Turkey), and capture a number of Byzantine forts. Heraclea Cybistra and Tyana fall to the Arabs.
Fall – Muslim Arabs reinvade Sicily, and lay siege to Palermo. Symeon, Byzantine commander of the imperial bodyguard (spatharios), surrenders the city in exchange for an safe departure.
Emperor Louis the Pious is reinstated as sole ruler of the Frankish Empire. He promises his sons Pepin I and Louis the German a greater share of the inheritance. His eldest son Lothair I is pardoned, but disgraced and banished to Italy.
February – Empress Judith stands trial to "undergo the judgment of the Franks" for a assembly arranged by Louis the Pious. She is exiled and send to the convent of St. Radegund at Poitiers.
Omurtag, ruler (khan) of the Bulgarian Empire, dies after a 17-year reign. He is succeeded by his youngest son Malamir, because his older brother Enravota favours Christianity.
Nominoe, duke of Brittany, is designated missus imperatoris (imperial emissary) by Louis the Pious, at Ingelheim (modern Germany).
A Uyghur Turk sues the son of a Chinese general, who had failed to repay a debt of 11 million government-issued copper coins. Emperor Wen Zong hears the news, and is so upset that he not only banishes the general, but attempts to ban all trade between Chinese and foreigners except for goods and livestock. This ban is unsuccessful, and trade with foreigners resumes, especially in maritime affairs overseas.
Summer – Ansgar, Frankish missionary, founds the first church at Birka (modern Sweden).
Ansgar is consecrated; he travels to Rome to receive the pallium from Pope Gregory IV.
Wang Chucun, general of the Tang Dynasty (d. 895)
July 10 – Zubaidah bint Ja`far, Abbasid princess
December 26 – Euthymius of Sardis, Byzantine monk and bishop
Omurtag, ruler (khan) of the Bulgarian Empire
Sadyrnfyw, Welsh bishop (approximate date)
Xue Tao, female Chinese poet (b. 768)
Yuan Zhen, politician of the Tang Dynasty (b. 779)
Year 831 (DCCCXXXI) was a common year starting on Sunday (link will display the full calendar) of the Julian calendar.