1256 (MCCLVI) was a leap year starting on Saturday of the Julian calendar, the 1256th year of the Common Era (CE) and Anno Domini (AD) designations, the 256th year of the 2nd millennium, the 56th year of the 13th century, and the 7th year of the 1250s decade. As of the start of 1256, the Gregorian calendar was 7 days ahead of the Julian calendar, which was the dominant calendar of the time.

• May 4 – Pope Alexander IV issues a papal bull Licet ecclesiae catholicae constituting the Augustinian monastic order at Lecceto Monastery.
• The city of Lviv, in present-day Ukraine, is founded by Danylo King of Rus.
• Theodore II Lascaris, Byzantine Emperor (in exile in the Empire of Nicaea), successfully concludes a military campaign started a year earlier to recover Thrace from the Bulgarians.

Abingdon School is founded. Roger Bacon becomes a Franciscan Friar.

• October – Mongol commander Baiju (operating under Hulagu Khan's command) leads his forces in a victory over Kay Ka'us II of the Sultanate of Rüm, thereby capturing Anatolia.
• December 15 – Hulagu Khan captures and destroys the Hashshashin stronghold at Alamut in present-day Iran.
• Hulagu Khan establishes the Ilkhanate dynasty of Persia, which will become one of four main divisions of the Mongol Empire.
• The Japanese Kenchō era ends, and the Kōgen era begins.

• January 6 – Gertrude the Great, German mystic
• Robert, Count of Clermont, French founder of the House of Bourbon (died 1317)

• Ibn al-Banna, Arab Mathematician (died 1321)

• January 28 – William II, Count of Holland, King of Germany
• June 13 – Tankei, Japanese sculptor (born 1173)
• September 1 – Kujo Yoritsune, Japanese shogun (born 1218)
• October 14 – Kujo Yoritsugu, Japanese shogun (born 1239)
• William of Kilkenny, Lord Chancellor of England
• Johannes de Sacrobosco, English scholar