1121 (MCXXI) was a common year starting on Saturday of the Julian calendar, the 1121st year of the Common Era (CE) and Anno Domini (AD) designations, the 121st year of the 2nd millennium, the 21st year of the 12th century, and the 2nd year of the 1120s decade. As of the start of 1121, the Gregorian calendar was 7 days ahead of the Julian calendar, which was the dominant calendar of the time.

• Concordat of Worms condemns Pierre Abélard's writings on the Holy Trinity. Later in the year, Canon Fulbert (not to be confused with Fulbert of Chartres (960-1028)), uncle of Heloise, has Abélard castrated.
• Reading Abbey is founded.
• David the Builder, King of Georgia, with the army of 55,000 defeats the 200,000-strong Muslim coalition troops at Didgori, Eastern Georgia.

• December 11 — Al-Afdal Shahanshah, Caliph of Egypt (b. 1066)
• William of Champeaux, French philosopher
• Zhou Tong, archery teacher and second military arts tutor of famous Song Dynasty General Yue Fei.