Year 831 (DCCCXXXI) was a common year starting on Sunday of the Julian calendar.

• 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 a 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"^[1] for an assembly arranged by Louis the Pious, and is exonerated.^{[clarification needed]}
• 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).^[2]

• An Uyghur Turk sues the son of a Chinese general, who had failed to repay a debt of 11 million government-issued copper coins. Emperor Wenzong 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, wife of caliph Harun al-Rashid, mother of al-Amin.
• December 26 – Euthymius of Sardis, Byzantine monk and bishop
• Omurtag, ruler (khan) of the Bulgarian Empire
• Sadyrnfyw, Welsh bishop (approximate date)
• Yuan Zhen, politician of the Tang Dynasty (b. 779)