1255 (MCCLV) was a common year starting on Friday of the Julian calendar, the 1255th year of the Common Era (CE) and Anno Domini (AD) designations, the 255th year of the 2nd millennium, the 55th year of the 13th century, and the 6th year of the 1250s decade. As of the start of 1255, the Gregorian calendar was 7 days ahead of the Julian calendar, which was the dominant calendar of the time.

• August – The final Cathar stronghold in southern France falls, eliminating the last refuge of the heretical Cathars after the Roman Catholic Church engaged in a program to crush the sect, starting in 1209 with the Albigensian Crusade.
• The death of Little Saint Hugh of Lincoln prompts persecution of Jews in England on accusations of blood libel.
• Lisbon becomes the capital of the Kingdom of Portugal.
• The Gothic cathedral at Bourges, France, is completed. It is now a UNESCO World Heritage Site.
• A survey of royal privileges is conducted, which is included in the Hundred Rolls, an English census seen as a follow up to the Domesday Book completed in 1086; the Hundred Rolls is later completed with two larger surveys in 1274/1275 and 1279/1280.

• 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.
• Königsberg (now Kaliningrad) is founded in Prussia.
• Theodore II Lascaris, 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 German state of Nassau is divided, not to be reunited until 1806.
• King Béla IV of Hungary grants Banská Bystrica municipal rights of a royal town.

• Hulagu Khan is dispatched by his brother Möngke Khan to destroy the remaining Muslim states in southwestern Asia.

• Albert I of Germany, Holy Roman Emperor (died 1308)
• William de Ros, 2nd Baron de Ros, claimant to the crown of Scotland (died 1317)
• Duccio di Buoninsegna, Sienese painter (died 1319)
• Grand Prince Andrey of Gorodets (approximate date; died 1304)
• Adolf of Nassau-Weilburg, King of the Romans (approximate date; died 1298)

• May 1 – Walter de Gray, English prelate and statesman
• Little Saint Hugh of Lincoln (born 1247)
• Jarler, Archbishop of Sweden
• Sundiata Keita, semi-historical hero and founder of the Mali Empire. (approximate date; born c. 1190)
• Batu Khan, Mongol ruler and founder of the Blue Horde.