1255

Millennium:	2nd millennium
Centuries:	12th century – 13th century – 14th century
Decades:	1220s 1230s 1240s – 1250s – 1260s 1270s 1280s
Years:	1252 1253 1254 – 1255 – 1256 1257 1258

Events

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.

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.