Year 834 (DCCCXXXIV) was a common year starting on Thursday of the Julian calendar.

• March 1 – Emperor Louis the Pious is restored^[1] as sole ruler of the Frankish Empire. After his re-accession to the throne, his eldest son Lothair I flees to Burgundy.
• Danish Vikings raid the trading settlement of Dorestad (present-day Wijk bij Duurstede), located in the southeast of the province of Utrecht (modern Netherlands).
• Summer – The Viking ship of Oseberg near Tønsberg (modern Norway) is buried in a mound, during the Viking Age (approximate date).
• The first mention is made of the Jona River ('the cold one') in Switzerland (approximate date).

• King Óengus II dies after a 14-year reign. He is succeeded by his nephew Drest IX, as ruler of the Picts.

• July 20 – Ansegisus, Frankish abbot and advisor of former emperor Charlemagne, dies at Fontenelle Abbey in Normandy (or 833).

• Aud the Deep-Minded, Icelandic queen
• Euthymius I, Ecumenical Patriarch of Constantinople (d. 917)
• Lady Shuiqiu, wife of Qian Kuan (d. 901)
• Mo Xuanqing, Chinese scholar
• Pi Rixiu, Chinese poet (approximate date)
• Robert, Frankish nobleman (d. 866)
• Tan Quanbo, Chinese warlord (d. 918)

• July 20 or 833 – Ansegisus, Frankish abbot
• Adelchis I, duke of Spoleto (Italy)
• Cellach mac Brain, king of Leinster (Ireland)
• Fridugisus, Anglo-Saxon abbot (approximate date)
• Gaucelm, Frankish nobleman
• Nasr ibn 'Abdallah, Muslim governor
• Odo I, Frankish nobleman
• Óengus II, king of the Picts
• Robert III, Frankish nobleman (b. 800)
• Wang Chengyuan, Chinese general (b. 801)
• Wang Tingcou, general of the Tang dynasty
• William, Frankish nobleman