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|Millennium:||1st millennium BC|
|426 BC by topic|
|Gregorian calendar||426 BC|
|Ab urbe condita||328|
|Ancient Egypt era||XXVII dynasty, 100|
|- Pharaoh||Artaxerxes I of Persia, 40|
|Ancient Greek era||88th Olympiad, year 3|
|Balinese saka calendar||N/A|
|Chinese calendar||甲寅年 (Wood Tiger)|
2271 or 2211
— to —
乙卯年 (Wood Rabbit)
2272 or 2212
|Coptic calendar||−709 – −708|
|Ethiopian calendar||−433 – −432|
|- Vikram Samvat||−369 – −368|
|- Shaka Samvat||N/A|
|- Kali Yuga||2675–2676|
|Iranian calendar||1047 BP – 1046 BP|
|Islamic calendar||1079 BH – 1078 BH|
|Minguo calendar||2337 before ROC|
|Thai solar calendar||117–118|
−299 or −680 or −1452
— to —
−298 or −679 or −1451
Year 426 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Tribunate of Cincinnatus, Albinus, Fusus and Cossus (or, less frequently, year 328 Ab urbe condita). The denomination 426 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 Athenian leader Cleon and Athenian general Demosthenes revitalise the city's military and naval forces despite opposition from Nicias, a rich merchant and soldier, and his supporters.
- Demosthenes unsuccessfully besieges the Corinthian colony of Leukas. As a result, he does not return to Athens, fearing for his life. However when, later in the year, Ambracia invades Acarnania, and the Acarnanians seek help from Demosthenes, who is patrolling the Ionian Sea coast with twenty Athenian ships, he reaches the Athenian naval base in the Gulf of Corinth at Naupactus and secures it just in time to defend it against a large Spartan army from Delphi under Eurylochus which has come to assist the Ambraciots. Demosthenes defeats the Spartan army and Eurylochus is killed during the Battle of Olpae. The Acarnanians and Ambraciots then sign a peace treaty.
- An Athenian army under Nicias, Hipponicus and Eurymedon defeats a combined Tanagran and Theban army in the Battle of Tanagra.