1288 (MCCLXXXVIII) was a leap year starting on Thursday of the Julian calendar, the 1288th year of the Common Era (CE) and Anno Domini (AD) designations, the 288th year of the 2ndmillennium, the 88th year of the 13thcentury, and the 9th year of the 1280s decade. As of the start of 1288, the Gregorian calendar was 7 days ahead of the Julian calendar, which was the dominant calendar of the time.

• June 5 – John I of Brabant defeats the duchy of Guelders in the Battle of Worringen — one of the largest battles in Europe of the Middle Ages — thus winning possession of the duchy of Limburg. The battle also liberates the city of Cologne from rule by the Archbishopric of Cologne; it had previously been one of the major ecclesiastical principalities of the Holy Roman Empire.
• August 8 – Pope Nicholas IV proclaims a crusade against King Ladislaus IV of Hungary, who had lost credibility by favoring his semi-pagan Cuman subjects and in general refusing to conform to the social standards of western Europe.
• The Scots Parliament creates a law allowing women to propose marriage to men during leap years; men who refuse such proposals are required to pay a fine to the spurned bride-to-be.
• The oldest surviving bell in the clocks atop the dome of St. Peter's Basilica in Rome dates to 1288.
• Vietnamese general Tran Hung Dao sinks the entire fleet of an invading Yuan dynasty Mongol army by placing steel-tipped bamboo stakes in the Bach Dang River, near Halong Bay.
• The Japanese era Koan ends, and the Shōō era begins.