Year 888 (DCCCLXXXVIII) was a leap year starting on Monday (link will display the full calendar) of the Julian calendar.
- January 13 – Emperor Charles III (the Fat) dies at Neidingen, after having suffered repeat bouts of an illness that may have been epilepsy. The Frankish Empire is split again, and falls apart into separate kingdoms. Count Odo, the hero of the Siege of Paris, is elected king of the West Frankish Kingdom, and crowned at Compiègne by Walter, archbishop of Sens. Other Frankish noblemen support the 8-year-old Charles the Simple (the posthumous son of the late king Louis the Stammerer).
- October – Alan I (the Great), count of Vannes, and his rival Judicael, unite their forces to defeat the Vikings at Questembert (or 889). Judicael is killed, in a notable victory for the Bretons, with 15,000 Vikings crushed, some few 400 escaping to their ships. In command of a 'united' Breton force, Alan is able to drive the Vikings back to the Loire River. Alan becomes sole ruler of Brittany, and over the Frankish counties of Rennes, Nantes, Coutances and Avranches.
- October – Battle of Milazzo: the Aghlabids score a crushing victory over a Byzantine fleet off Sicily.
- Winter – King Arnulf of Carinthia leads an East Frankish expion into Italy, after he is recognized as overlord of France and Burgundy. Arnulf descends with an army over the Brenner Pass, and meets King Berengar I at a peace conference at Trento. Berengar grants him two counties in the Val d'Adige (Northern Italy), and does homage to Arnulf as overlord. In turn, Arnulf confirms Berengar as king of Lombardia, and returns to Germany.
- Lord Æthelred II of the Mercians is struck down with a debilitating illness. His wife, Princess Æthelflæd (a daughter of Alfred the Great) of Wessex, joins him as joint ruler of Mercia (approximate date).
- January 13 – Charles the Fat, Frankish emperor (b. 839)
- April 20 – Xi Zong, emperor of the Tang Dynasty (b. 862)
- June 11 – Rimbert, archbishop of Bremen (b. 830)
- June 30 – Æthelred, archbishop of Canterbury
- Áed mac Conchobair, king of Connacht (Ireland)
- Æthelswith, Anglo-Saxon queen
- Al-Mundhir, Muslim emir of Córdoba
- Cerball mac Dúnlainge, king of Osraige (Ireland)
- Ingelger, founder of the House of Anjou
- Judicael, duke of Brittany (or 889)
- Le Yanzhen, Chinese warlord
- Nasra of Tao-Klarjeti, Georgian prince
- Sichfrith mac Ímair, king of Dublin
- Tetbert, Frankish nobleman
- Zhang Gui, Chinese warlord
- Zhou Bao, Chinese general (b. 814)
- ^ Gwatking, H. M., Whitney, J. P., et al. Cambridge Medieval History: Volume III–Germany and the Western Empire. Cambridge University Press: London (1930).