1251

1251 (MCCLI) was a common year starting on Sunday of the Julian calendar, the 1251st year of the Common Era (CE) and Anno Domini (AD) designations, the 251st year of the 2nd millennium, the 51st year of the 13th century, and the 2nd year of the 1250s decade. As of the start of 1251, the Gregorian calendar was 7 days ahead of the Julian calendar, which was the dominant calendar of the time.

• April – The first Shepherds' Crusade, a domestic French uprising in response to events in Egypt during the Seventh Crusade, occurs.
• December 26 – King Alexander III of Scotland marries Margaret, daughter of King Henry III of England, precipitating a power struggle between the two monarchs.
• Andrew de Longjumeau, dispatched two years earlier by King Louis IX of France as an ambassador to the Mongols, meets the king in Palestine, with reports from the Mongols and Tartary; his mission is considered a failure.
• Mindaugas of Lithuania is baptized, in prelude to his crowning as King of Lithuania in 1253.
• Alexander Nevsky signs the first peace treaty between Kievan Rus' and Norway.
• King Conrad IV of Germany invades Italy, but fails to subdue the supporters of Pope Innocent IV.
• The German city of Berlin, founded some fifty years earlier, receives its city charter.
• Ottokar II of Bohemia, later to become King of Bohemia, is elected Duke of Austria.
• The earliest known manuscript of The Proverbs of Alfred, a collection of sayings of England's Alfred the Great, is written.

• Möngke Khan is elected as the fourth great Khan of the Mongol Empire.
• Carving of the Tripitaka Koreana, a collection of Buddhist scriptures recorded on some 81,000 wooden blocks, is completed.

• Hōjō Tokimune, 8th regent of the Kamakura shogunate (died 1284)

• Bohemund V of Antioch
• Peter I, Duke of Brittany (born 1190)
• Eljigidei, Mongol commander of Persia
• Isobel of Huntingdon (born 1199)
• Matthias II, Duke of Lorraine