Year 311 (CCCXI) was a common year starting on Monday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Valerius and Maximinus (or, less frequently, year 1064 Ab urbe condita). The denomination 311 for this year has been used after the early mediaeval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
- May 5 – Emperor Galerius declares on his deathbed religious freedom and issues his Edict of Toleration, ending persecution of Christians in the Eastern part of the Roman Empire. He dies, age 51, from a gruesome disease, possibly bowel cancer or Fournier gangrene.
- Maximinus II divides the Eastern Empire between co-emperor Licinius, and mobilises his army for a campaign in Asia Minor. He recommences the persecution of Christians.
- Maxentius, Roman usurper, reconquers the African provinces from Domitius Alexander.
- Maxentius builds the Circus of Maxentius near the Via Appia. The circus is 513 metres long and 91 metres wide, and offers an accommodation to a few 10,000 people.
- December 3 – Diocletianus dies at his palace in Split (Croatia). Possibly he commits suicide.
- Jin Huaidi, emperor of the Jin Dynasty, is captured at Luoyang. The capital city is pillaged by Liu Cong, ruler of the Xiongnu state (Huns), the invaders slaughter 30,000 citizens.
- July 2 – Pope Miltiades succeeds Pope Eusebius as the 32nd pope.
- The Donatist schism occurs in the African church.