Year 325 (CCCXXV) was a common year starting on Friday of the Julian calendar. At the time, it was known as the Year of the Consulship of Proculus and Paulinus (or, less frequently, year 1078 Ab urbe condita). The denomination 325 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.

• Emperor Constantine I personally assures the security of the Danube frontier by defeating the Goths, the Vandals, and the Sarmatians (in previous years, see German and Sarmatian campaigns of Constantine).
• Licinius is executed in Thessalonica, on a charge of conspiring and raising troops against Constantine I.^[1]
• Gladiatorial combat is outlawed in the Roman Empire (Constantine forbade criminals being forced to fight to the death as gladiators, first ban was Honorius' in 399).

• April 1 – Crown Prince Cheng of Jin, age 4, succeeds his father Ming of Jin as emperor of the Eastern Jin dynasty. During his reign, he is largely advised by regents, his uncle Yu Liang and high-level officials.

• Colossus of Constantine, from the Basilica of Maxentius and Constantine, Rome, is started to be made (alternatively in 312-315). It is now kept at Palazzo dei Conservatori, Rome.

• May 20 – First Council of Nicaea: Constantine I summons an ecumenical council of bishops in Nicaea (Turkey). The Nicene Creed, adopted on June 19, declares that the members of the Trinity (the Father, the Son, and the Holy Spirit) are equal. The council decides that Easter is celebrated on the first Sunday after the first full moon after the vernal equinox. Arius is exiled to Illyria; his works are confiscated and consigned to the flames.
• The Church of the Nativity is built in Bethlehem (in 326-339).

• Ammianus Marcellinus, Roman historian (approximate date)
• Procopius, Roman general and usurper (approximate date)
• Wang Meng (or Jinglüe), Chinese prime minister (d. 375)

• October 18 – Ming of Jin, Chinese emperor (b. 299)
• Iamblichus, Syrian philosopher and writer (b. 245)
• Licinius, Roman consul and emperor (executed)
• Li Ju (or Shihui), Chinese general and warlord
• Sextus Martinianus, Roman Emperor (executed)
• Tuoba Heru, Chinese prince of the Tuoba Dai