1240 (MCCXL) was a leap year starting on Sunday of the Julian calendar, the 1240th year of the Common Era (CE) and Anno Domini (AD) designations, the 240th year of the 2nd millennium, the 40th year of the 13th century, and the 1st year of the 1240s decade. As of the start of 1240, the Gregorian calendar was 7 days ahead of the Julian calendar, which was the dominant calendar of the time.

• Batu Khan and the Golden Horde sack the Ruthenian city of Kiev.
• Mandinka prince Sundiata Keita defeats Sosso king Soumaoro Kanté at the Battle of Kirina, beginning the Mali Empire.
• July 15 – Russian prince Alexander Nevsky defeats the Swedes in the Battle of the Neva, saving Russia from a full-scale enemy invasion from the North.
• End of the civil war era in Norway.
• Saint Maurice is started to be portrayed as Moor.

• Abulafia, Maltese Jewish philosopher (died 1292)
• Pope Benedict XI (died 1304)
• Stephen V of Hungary (died 1272)
• Sigerus of Brabant, French theologian (died 1284)
• Albert the Degenerate, landgrave of Thuringia (approximate date; died 1314)

• April 11 – Llywelyn the Great, King of Gwynedd
• Ibn Arabi, Andalusian Arab philosopher
• Jacques de Vitry, French historian and theologian (born c. 1180)
• William de Warenne, 6th Earl of Surrey (born 1166)

• Aragon – James I King of Aragon and count of Barcelona (reigned from 1213 to 1276)
• Castile – Ferdinand III, the Saint King of Castile and León (reigned from 1217 to 1252)