Year 121 (CXXI) was a common year starting on Tuesday of the Julian calendar. At the time, it was known as the Year of the Consulship of Verus and Augur (or, less frequently, year 874 Ab urbe condita). The denomination 121 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.

• Roman settlement in present-day Wiesbaden, Germany, is first mentioned.
• Emperor Hadrian fixes the border between Roman Britain and Caledonia, on a line running from the River Tyne to the Solway Firth.
• Construction of the Temple of Venus and Roma begins in Rome.

• Era name changes from Yongning (2nd year) to Jianguang in the Chinese Eastern Han dynasty.^{(Needs clarification or deletion)}

• April 26 – Marcus Annius Verus, later Emperor Marcus Aurelius (d. 180)

• Cai Lun, Chinese inventor of paper and the papermaking process (b. AD 50)^[1]
• Deng Sui, Chinese empress of the Han dynasty (b. AD 81)^[2]
• Eleutherius and Antia, Roman Christian martyrs and saints