| Centuries: | 6th century BC - 5th century BC - 4th century BC |
| Decades: | 450s BC 440s BC 430s BC - 420s BC - 410s BC 400s BC 390s BC |
| Years: | 424 BC 423 BC 422 BC - 421 BC - 420 BC 419 BC 418 BC |
| 421 BC by topic | |
| Politics | |
| State leaders - Sovereign states | |
| Birth and death categories | |
| - | |
| Establishments and disestablishments categories | |
| Establishments - | |
| Gregorian calendar | 421 BC |
| Ab urbe condita | 333 |
| Armenian calendar | N/A |
| Bahá'í calendar | -2264 – -2263 |
| Berber calendar | 530 |
| Buddhist calendar | 124 |
| Burmese calendar | -1058 |
| Chinese calendar | 2216/2276 ([[Sexagenary cycle|]]年) — to —
2217/2277([[Sexagenary cycle|]]年) |
| Coptic calendar | -704 – -703 |
| Ethiopian calendar | -428 – -427 |
| Hebrew calendar | 3340 – 3341 |
| Hindu calendars | |
| - Vikram Samvat | -365 – -364 |
| - Shaka Samvat | N/A |
| - Kali Yuga | 2681 – 2682 |
| Holocene calendar | 9580 |
| Iranian calendar | 1042 BP – 1041 BP |
| Islamic calendar | 1074 BH – 1073 BH |
| Japanese calendar | |
| - Imperial Year | Kōki 240 (皇紀240年) |
| Julian calendar | -375 |
| Korean calendar | 1913 |
| Thai solar calendar | 123 |
Events
By place
Greece
- 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 prewar situation: most wartime gains are to be returned. Seventeen representatives from each side swear an oath to uphold the treaty, which is meant to last for fifty years. 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.
Italy
By topic
Art
- The construction of the Porch of the Maidens (the Caryatid Porch) commences at the Erechtheion which is part of the Acropolis in Athens.
Drama
- Aristophanes' play The Peace is performed.
Births
Deaths
- Cratinus, Greek comedy author (approximate date) (b. 520 BC)
