1252

1252 (MCCLII) was a leap year starting on Monday of the Julian calendar, the 1252nd year of the Common Era (CE) and Anno Domini (AD) designations, the 252nd year of the 2nd millennium, the 52nd year of the 13th century, and the 3rd year of the 1250s decade. As of the start of 1252, the Gregorian calendar was 7 days ahead of the Julian calendar, which was the dominant calendar of the time.

• May 15– Pope Innocent IV issues the papal bull Ad exstirpanda, which allows the torture of heretics in the Medieval Inquisition. Torture is soon used widely across Catholic Europe.
• The first European gold coins are minted in the Italian city of Florence, and are known as florins.
• The Polish land of Lebus is made a part of the German state of Brandenburg, beginning Brandenburg's expansion into previously Polish areas (Neumark).
• The Swedish city of Stockholm is founded by Birger Jarl.
• The Lithuanian city of Klaipėda is founded by the Teutonic Knights.
• The town and monastery of Orval in Belgium burns to the ground; rebuilding takes 100 years.
• Thomas Aquinas travels to the University of Paris to begin his studies there for a master's degree.
• In astronomy, work begins on the recording of the Alfonsine tables.