Year 291 (CCXCI) was a common year starting on Thursday of the Julian calendar. At the time, it was known in Rome as the Year of the Consulship of Tiberianus and Dio (or, less frequently, year 1044 Ab urbe condita). The denomination 291 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.

• Winter: The emperors Diocletian and Maximian convene in Milan.^[1]
• An uneasy peace is established between the emperors Diocletian and Maximian on the one hand, and the rival emperor Carausius on the other.
• Perhaps in cooperation with the forces of Maximian, Carausius campaigns successfully against Germanic raids in Gaul and Britain. Also during his reign, Carausius begins building the forts of the Saxon Shore.

• The Alemanni, having been expelled from part of their territory by the Burgundians, seek to regain their lost lands. These peoples had unsuccessfully invaded Gaul in tandem in 285/6, and the Alemanni had likely been weakened by the Roman counter-invasions of 287 and 288.
• A force of Goths defeat the Burgundians.
• The Tervingian Goths and Taifali fight the Vandals and Gepids.

• The Blemmyes invade the Kingdom of Kush.

• King Bahram II fights against a coalition consisting of his brother Hormizd of Sakastan, the Kushano-Sasanians, and the Gilans.

• War of the Eight Princes: After the death of Emperor Sima Yan (Jin Wudi), a civil war breaks out among the princes and dukes of the Jin Dynasty. The struggle devastates and depopulates the provinces of northern China.

• Saint Agnes, Christian martyress (d. c. 304)
• Saint Hilarion, anchorite and saint (d. 371)
• Li Xiu, female general during the Jin Dynasty
• Saint Philomena, Christian martyress (d. c. 304)

• Sima Liang, regent during the reign of Sima Yan
• Sima Wei, prince during the Jin Dynasty (b. 271)
• Wei Guan, general of the Kingdom of Wei (b. 220)
• Wen Yang, general of the Kingdom of Wei (b. 238)
• Yang Jun, official during the reign of Sima Yan