428 (CDXXVIII) was a leap year starting on Sunday of the Julian calendar, the 428th year of the Common Era (CE) and Anno Domini (AD) designations, the 428th year of the 1st millennium, the 28th year of the 5th century, and the 9th year of the 420s decade. As of the start of 428, the Gregorian calendar was 1 day ahead of the Julian calendar, which was the dominant calendar of the time.

• Bonifacius, Roman governor of Africa, asks for help from the Vandals in his revolt.
• Flavius Felix is elected consul for the Western Empire.

• King Gunderic, age 49, dies after a reign of 21 years. He is succeeded by his half-brother Genseric.
• King Vortigern asks Germanic warriors to help him strengthen his position in Britain. He hires Saxons as mercenaries to fight against the Picts and the Scots beyond Hadrian's Wall.
• Chlodio, king of the Salian Franks, invades Northern Gaul. He defeats the Roman army at Cambrai. He extends his kingdom south to the River Somme. Tournai (modern Belgium) becomes his residence.

• Artaxias IV, last king of Greater Armenia, is deposed by Bahram V. The Arshakuni Dynasty ends and the kingdom becomes a province of the Persian Empire.

• October 26 – The planet Venus occults the planet Jupiter.

• April 10 – Nestorius is made patriarch of Constantinople. He preaches a new doctrine that will be called Nestorianism. It comes under immediate attack from pope Celestine I and Cyril of Alexandria.
• Hydatius becomes bishop of Aquae Flaviae in Gallaecia (modern Chaves) in Portugal.
• John succeeds Theodotus as patriarch of Antioch, and gives his support to Nestorius.
• Euthymius the Great builds a monastery in Palestine, near the Dead Sea.

• Tuoba Huang, prince of the Xianbei state Northern Wei (d. 451)

• Gunderic, king of the Vandals and Alans (b. 379)
• Qifu Chipan, prince of the Xianbei state Western Qin
• Theodore of Mopsuestia, bishop and theologian

• Traina, Giusto (2011). 428 AD: An Ordinary Year at the End of the Roman Empire. Princeton University Press. ISBN 978-0-691-15025-3.