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Year 1081 (MLXXXI) was a common year starting on Friday (link will display the full calendar) of the Julian calendar.
- April 1 – Byzantine emperor Nikephoros III Botaneiates is overthrown by Alexios I Komnenos, who is crowned on April 5, ending the Middle Byzantine period, and beginning the Comnenan Dynasty.
- May 8 – Alfonso VI of Castile marries Constance of Burgundy.
- October 18 – Byzantine–Norman wars: Battle of Dyrrhachium: Alexios I helps defend Albania from the Normans (the first recorded mention of Albania), but is defeated by Robert Guiscard, Norman Duke of Apulia and Calabria, outside the city of Dyrrhachium (Durrës), the Byzantine capital of Illyria. The Normans govern Albania from southern Italy until around 1100.
- Corfu is taken from the Byzantine Empire by Robert Guiscard.
- Turkish emir Tzachas conquers İzmir, and founds a short-lived principality, which emerges as the first sea power in Turkish history.
- Battle of Mynydd Carn (near St. David's in Wales): Gruffudd ap Cynan, in alliance with Rhys ap Tewdwr, (prince of Deheubarth) defeats the forces of Trahaearn ap Caradog, Caradog ap Gruffydd and Meilir ap Rhiwallon (who are all killed), allowing Gruffudd to claim the Kingdom of Gwynedd.
- Construction begins on St. Canute's Cathedral in Odense, Denmark.
- Pope Gregory VII writes a letter to the Bishop of Metz, about Henry IV's behavior.