Year 1255 (MCCLV) was a common year starting on Friday (link will display the full calendar) of the Julian calendar.
Hulagu Khan is dispatched by his brother Möngke Khan to destroy the remaining Muslim states in southwestern Asia.
May – William of Rubruck from Constantinople returns to Cyprus from his missionary journey to convert the Tatars of central and eastern Asia, his efforts having been unsuccessful.
August – The final Cathar stronghold in southern France falls, eliminating their last refuge since the Roman Catholic Church began the Albigensian Crusade to crush the sect in 1209.
The death of Little Saint Hugh of Lincoln prompts the persecution of Jews in England, based on the blood libel.
Lisbon becomes the capital of Portugal.
A survey of royal privileges is conducted, which is included in the Hundred Rolls, an English census seen as a followup to the Domesday Book completed in 1086; the Hundred Rolls is later completed with two larger surveys in 1274/1275 and 1279/1280.
Königsberg (now Kaliningrad) is founded by the Teutonic Knights in Prussia and named in honour of king Ottokar II of Bohemia.
Theodore II Laskaris, Byzantine Emperor (in exile in the Empire of Nicaea), conducts a military campaign to recover Thrace from the Bulgarians. He concludes the task successfully a year later in 1256.
The Duchy of Bavaria is split into Upper and Lower Bavaria.
The lands of the House of Nassau are divided, not to be reunited until 1806.
King Béla IV of Hungary grants Banská Bystrica the municipal rights of a royal town.
Arts and culture
The Gothic cathedral at Bourges, France, is completed. It is now a UNESCO World Heritage Site.
At the death of Bernardo Bonsignori, his brother, Orlando, is left sole director of the largest banking firm in western Europe, the Gran Tavola of Siena.
July – Albert I of Germany, Holy Roman Emperor (d. 1308)
William de Ros, 2nd Baron de Ros, claimant to the crown of Scotland (d. 1317)
Duccio di Buoninsegna, Sienese painter (d. 1319)
Grand Prince Andrey of Gorodets (approximate date; d. 1304)
Adolf of Nassau-Weilburg, King of the Romans (approximate date; d. 1298)
May 1 – Walter de Gray, English prelate and statesman
August 27 – Little Saint Hugh of Lincoln (b. 1247)
Batu Khan, Mongol ruler and founder of the Blue Horde
Jarler, Archbishop of Uppsala since 1236
Sundiata Keita, semi-historical hero and founder of the Mali Empire (approximate date; b. c. 1190)
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