421 BC

• Nicias, the leader of the aristocratic and peace party in Athens and Pleistoanax, King of Sparta, negotiate the Peace of Nicias between Athens and Sparta, which brings a temporary end to the Peloponnesian War. The essence of the Peace of Nicias is a return to the antebellum period with most wartime gains being returned. Seventeen representatives from each side swear an oath to uphold the treaty, which is meant to last for one generation (30 years: meaning they are not responsible for the next generation's decision). All of Sparta's allies agree to sign the peace except for the Boeotians, Corinth, Elis, and Megara.
• Alcibiades engineers an anti-Spartan alliance between Athens and the democracies of Argos, Mantinea and Elis.

• The city of Cumae, the most northerly of the Greek colonies in Italy, falls to the Samnites.

• The construction of the Porch of the Maidens (the Caryatid Porch) commences at the Erechtheion which is part of the Acropolis in Athens.{{Cite journal |last=Randall |first=Richard H. |date=1953 |title=The Erechtheum Workmen |url=https://www.jstor.org/stable/500060 |journal=American Journal of Archaeology |volume=57 |issue=3 |pages=199–210 |doi=10.2307/500060 |jstor=500060 |issn=0002-9114|url-access=subscription }}

• Aristophanes' play Peace is performed.{{Cite journal |last=May |first=Regine |date=2008 |title=Aristophanes in Performance 421 BC-AD 2007: 'Peace', 'Birds', and 'Frogs' by Edith Hall, Amanda Wrigley (review) |url=https://muse.jhu.edu/pub/427/article/824866 |journal=Modern Language Review |volume=103 |issue=3 |pages=807–808 |doi=10.1353/mlr.2008.0126 |issn=2222-4319|url-access=subscription }}

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• Cratinus, Greek comedy author (approximate date) (b. 520 BC)

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