Year 425 (CDXXV) was a common year starting on Thursday of the Julian calendar. At the time, it was known as the Year of the Consulship of Theodosius and Valentinianus (or, less frequently, year 1178 Ab urbe condita). The denomination 425 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.

• Roman civil war: Summer – Joannes, Roman usurper, is defeated at the fortified city of Ravenna and brought to Aquileia. After a humiliating parade on a donkey and the insults of the populace, he is executed.
• October 23 – Valentinian III, six-year-old son of Galla Placidia, is installed as emperor (Augustus) of the Western Roman Empire. Real power is in the hands of his mother who becomes a regent.
• Flavius Aetius leads a force of Huns (60,000 men) into Northern Italy. He reaches a compromise with Placidia, in return for obtaining the rank commander-in-chief (magister militum) in Gaul.
• The Huns advance unopposed on Constantinople, but are halted by a plague that decimates their hordes (see 433).^[1]

• After death of Gamaliel VI, the Sanhedrin is disbanded by the Roman Empire.

• Last known usage of Demotic script in Egypt (in 452).
• c. 425–426 – Mausoleum of Galla Placidia is built.

• February 27 – The University of Constantinople is founded by emperor Theodosius II at the urging of his wife Aelia Eudocia.

• Buddhism begins to spread to Southeast Asia.

• Iamblichus, Syrian philosopher
• Zeno, Byzantine Emperor (approximate date)

• November 5 – Atticus, archbishop of Constantinople
• Gamliel VI, last Nasi (head of the Sanhedrin)
• Helian Bobo, emperor of the Chinese Xiongnu state Xia (born 381)
• Joannes, Roman usurper
• Mavia, Arab warrior-queen
• Sulpicius Severus, Christian writer (approximate date)
• Yax Nuun Ayiin I 15th Ajaw of Tikal (approximate date)