Year 420 (CDXX) was a leap year starting on Thursday of the Julian calendar. At the time, it was known as the Year of the Consulship of Theodosius and Constantius (or, less frequently, year 1173 Ab urbe condita). The denomination 420 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.

• the Franks cross the Rhine and invade Northern Gaul. In Italia an army is prepared to campaign with Castinus as chief.

• Yazdegerd I dies after a 21-year reign, and is succeeded by his son Bahram V, who becomes head of the Persian Empire.
• Abdas, bishop of Susa, is accused of burning down one of the fire temples of Ahura Mazda, and after refusing to rebuild it, he is executed, under orders of Shah Yazdegerd I.

• July 10 – The Jin Dynasty ends in China. Liu Yu (Emperor Wu of Liu Song) becomes the first ruler of the Liu Song Dynasty. Nanjing is reinstated as the capital of northern China.^[1]
• The Southern Dynasties begin in China.^[1]
• Guisin becomes king of the Korean kingdom of Baekje.^[2]
• The Gupta Empire reaches its peak territorial extent under Chandragupta II and his son Kumaragupta I.

• Anthemius, emperor of the Western Roman Empire
• Ecdicius, Roman general (magister militum)
• Libius Severus, emperor of the Western Roman Empire
• Majorian, emperor of the Western Roman Empire
• Palladius, caesar and son of Petronius Maximus
• Valamir, king of the Ostrogoths (he is also thought to have been born in 425 AD^[3])
• Yuan Can, high official of the Liu Song Dynasty (d. 477)

• January 21 – Yazdegerd I, king of the Sassanid Empire
• February 26 – Saint Porphyry, bishop of Gaza (Palestine)
• September 30 – Saint Jerome, priest and translator of the Bible
• Saint Abdas, bishop of Susa (Iran)
• Li Xin, duke of the Chinese state Western Liang
• Orosius, Christian historian and theologian (approximate date)
• Pelagius, British monk (approximate date)
• Yao, empress consort and wife of Mingyuan