|Millennium:||1st millennium BC|
|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|
|Gregorian calendar||421 BC|
|Ab urbe condita||333|
|Ancient Egypt era||XXVII dynasty, 105|
|- Pharaoh||Darius II of Persia, 3|
|Ancient Greek era||89th Olympiad, year 4|
|Chinese calendar||己未年 (Earth Goat)
2276 or 2216
— to —
庚申年 (Metal Monkey)
2277 or 2217
|Coptic calendar||−704 – −703|
|Ethiopian calendar||−428 – −427|
|- Vikram Samvat||−364 – −363|
|- Shaka Samvat||N/A|
|- Kali Yuga||2681–2682|
|Iranian calendar||1042 BP – 1041 BP|
|Islamic calendar||1074 BH – 1073 BH|
|Minguo calendar||2332 before ROC
|Thai solar calendar||122–123|
|Wikimedia Commons has media related to 421 BC.|
Year 421 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Consulship of Vibulanus and Barbatus (or, less frequently, year 333 Ab urbe condita). The denomination 421 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.
- 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 thirty years or one generation (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 construction of the Porch of the Maidens (the Caryatid Porch) commences at the Erechtheion which is part of the Acropolis in Athens.