|Covid-19|January 20 – In Westminster, the first elected English parliament (called Montfort's Parliament) conducts its first meeting in the Palace of Westminster, later to be known as the Houses of Parliament.
May 28 – Future King Edward I of England escapes the captivity of Simon de Montfort, 6th Earl of Leicester.
August 4 – The Battle of Evesham of the Second Barons' War is fought in Worcestershire, with the army of Edward defeating the forces of rebellious barons led by Simon de Montfort, resulted in the death of Montfort and many of his allies. This was sometimes considered the death of chivalry in England.
Reconquista: King Alfonso X of Castile captures the city of Alicante, Spain from the Moors.
The Isle of Man comes under Scottish rule.
Mongol armies, led by Nogai Khan, raid Thrace.
In the first major battle in 5 years since the Song dynasty Chinese pushed the forces of Kublai Khan back across the Yangzi River after Möngke Khan's failed invasion in 1259, Kublai Khan engages the Chinese in Sichuan province. Kublai gains a preliminary victory and war booty of 146 captured Song Dynasty naval ships.
The Book of Aneirin, a Welsh manuscript of poetry, is penned.
The brewing of Budweiser Budvar beer begins in Bohemia; Budweiser Budvar has been produced continuously there to this day.
Correspondence from Pope Clement IV contains the first known mention of the ring of the Fisherman, an item of papal regalia then used to seal personal correspondence from the pope and later for papal bulls.
February 5 – Pope Clement IV succeeds Pope Urban IV as the 183rd pope.
The Mamluk Sultanate Bahri dynasty of Egypt captures several cities and towns from Crusader states in the Middle East, including the cities of Haifa, Arsuf, and Caesarea Maritima; these events eventually precipitate the Eighth Crusade in 1267.
Kublai Khan sends a delegation to Japan, which loots islands along the way.
Fire destroys parts of Old Cairo.
India, Delhi: Ghiyas-Ud-Din-Balban comes to the throne and introduces Sijdah.
May 10 – Emperor Fushimi of Japan (d. 1317)
King Alfonso III of Aragon
approximate date – Dante Alighieri, Italian poet (d. 1321)
approximate date – Maria de Molina, (d. 1321)
January 20 – John Maunsell, Lord Chancellor of England
February 8 – Hulagu Khan of the Mongol Empire (b. 1217)
May 16 – Saint Simon Stock, English saint (b. c. 1165)
August 4 (Killed in the Battle Of Evesham)
Hugh le Despencer, 1st Baron le Despencer (b. 1223)
Henry de Montfort (b. 1238)
Peter de Montfort
Simon de Montfort, 6th Earl of Leicester (b. 1208)
December 3 – Odofredus, Italian jurist
Al-Abharī, Persian philosopher and mathematician (b. 1200)
Year 1265 (MCCLXV) was a common year starting on Thursday (link will display the full calendar) of the Julian calendar.