311 (CCCXI) was a common year starting on Monday of the Julian calendar, the 311th year of the Common Era (CE) and Anno Domini (AD) designations, the 311th year of the 1st millennium, the 11th year of the 4th century, and the 2nd year of the 310s decade. As of the start of 311, the Gregorian calendar was 1 day ahead of the Julian calendar, which was the dominant calendar of the time.

• May 5 – Emperor Galerius says there is religious freedom . He issues his Edict of Toleration, ending persecution of Christians in the Eastern part of the Roman Empire. He dies at age 51.
• Maximinus II divides the Eastern Empire. He starts to persecute Christians again.
• Maxentius, Roman usurper, takes the African provinces from Domitius Alexander.
• Maxentius builds the Circus of Maxentius near the Via Appia.
• December 3 – Diocletianus dies in Split, Croatia.

• Jin Huaidi, emperor of the Jin Dynasty, is captured at Luoyang. The capital city is looted by Liu Cong, ruler of the Xiongnu state (Huns).

• July 2 – Pope Miltiades succeeds Pope Eusebius as the 32nd pope.
• The Donatist split occurs in the African church.

• Domitius Alexander, Roman usurper (approximate date)
• December 3 – Gaius Aurelius Valerius Diocletianus, Roman Emperor
• May 5 – Gaius Galerius Valerius Maximianus, Roman emperor
• November 25 – Saint Peter, Patriarch of Alexandria (martyred)
• Sima Yue, prince and regent of the Jin Dynasty