Year 421 (CDXXI) was a common year starting on Saturday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Agricola and Eustathius (or, less frequently, year 1174 Ab urbe condita). The denomination 421 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.

This article relies largely or entirely on a single source. (March 2019)

• September 2 – Constantius III, emperor of the Western Roman Empire^[2]
• Jin Gongdi, last emperor of the Jin Dynasty (b. 386)
• Li Xun, ruler of the Chinese state Western Liang
• Mary of Egypt, patron saint (approximate date)
• Ravina I, rabbi (teacher) and Jewish Talmudist