|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:||426 BC 425 BC 424 BC – 423 BC – 422 BC 421 BC 420 BC|
|423 BC by topic|
|Gregorian calendar||423 BC|
|Ab urbe condita||331|
|Ancient Egypt era||XXVII dynasty, 103|
|- Pharaoh||Darius II of Persia, 1|
|Ancient Greek era||89th Olympiad, year 2|
|Chinese calendar||丁巳年 (Fire Snake)
2274 or 2214
— to —
戊午年 (Earth Horse)
2275 or 2215
|Coptic calendar||−706 – −705|
|Ethiopian calendar||−430 – −429|
|- Vikram Samvat||−366 – −365|
|- Shaka Samvat||N/A|
|- Kali Yuga||2679–2680|
|Iranian calendar||1044 BP – 1043 BP|
|Islamic calendar||1076 BH – 1075 BH|
|Minguo calendar||2334 before ROC
|Thai solar calendar||120–121|
|Wikimedia Commons has media related to 423 BC.|
Year 423 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Consulship of Atratinus and Ambustus (or, less frequently, year 331 Ab urbe condita). The denomination 423 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.
- Ochus, satrap of Hyrcania and son of Artaxerxes I and a Babylonian concubine, seizes the Persian throne from his half brother Secydianus (or Sogdianus), whom he has executed. The new king rules as Darius II.
- The Athenian general, Laches, successfully moves in the Athenian Assembly for an armistice with Sparta to check the progress of Sparta's most effective general, Brasidas. However, the "Truce of Laches" has little impact on Brasidas and collapses within a year.
- Brasidas ignores the proposed year-long truce and proceeds to take Scione and Mende in the hope of reaching Athens and freeing Spartan prisoners. Athens sends reinforcements under Nicias who retakes Mende.
- Aristophanes' play The Clouds is performed as is Sophocles' play Maidens of Trachi and The Putine (The Bottle), by Cratinus.