Year 291 (CCXCI) was a common year starting on Thursday (link will display the full calendar) 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 Dominicalendar era became the prevalent method in Europe for naming years.
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.
Northern Europe
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.
King Bahram II fights against a coalition consisting of his brother Hormizd of Sakastan, the Kushano-Sasanians, and the Gilans.
China
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.
