Year 420 BC was a year of the pre-Julian Roman calendar. The denomination 420 BC 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 young and popular Alcibiades is elected "Strategos" (one of a board of ten generals) and begins to dominate Athenian life and politics. A Quadruple Alliance of Athens, Argos, Mantineia and Elis, which has been organised by Alcibiades (in opposition to Nicias) confronts a Spartan-Boeotian alliance.
• Around 420 BC, Spartans were excluded from participating in the Olympic Games due to their ongoing conflicts with other Greek city-states, violating a peace treaty. This exclusion was part of a broader effort to maintain the peaceful atmosphere of the games.^[1]

• Euripides' play The Suppliant Women is performed.
• Euripides' play Phaethon is performed.

• Protagoras, Greek presocratic philosopher (b. c. 490 BC)