Year 331 BC was a year of the pre-Julian Roman calendar.
|Millennium:||1st millennium BC|
|Centuries:||5th century BC – 4th century BC – 3rd century BC|
|Decades:||360s BC 350s BC 340s BC – 330s BC – 320s BC 310s BC 300s BC|
|Years:||334 BC 333 BC 332 BC – 331 BC – 330 BC 329 BC 328 BC|
|331 BC by topic|
|State leaders – Sovereign states|
|Birth and death categories|
|Births – Deaths|
|Establishments and disestablishments categories|
|Establishments – Disestablishments|
|Gregorian calendar||331 BC|
|Ab urbe condita||423|
|Ancient Egypt era||XXXII dynasty, 2|
|- Pharaoh||Alexander the Great, 2|
|Ancient Greek era||112th Olympiad, year 2|
|Balinese saka calendar||N/A|
|Chinese calendar||己丑年 (Earth Ox)|
2366 or 2306
— to —
庚寅年 (Metal Tiger)
2367 or 2307
|Coptic calendar||−614 – −613|
|Ethiopian calendar||−338 – −337|
|- Vikram Samvat||−274 – −273|
|- Shaka Samvat||N/A|
|- Kali Yuga||2770–2771|
|Iranian calendar||952 BP – 951 BP|
|Islamic calendar||981 BH – 980 BH|
|Minguo calendar||2242 before ROC|
|Thai solar calendar||212–213|
−204 or −585 or −1357
— to —
−203 or −584 or −1356
|Wikimedia Commons has media related to 331 BC.|
- Alexander leaves Egypt and leads his forces towards Phoenicia. He leaves Cleomenes of Naucratis as the ruling nomarch to control Egypt.
- October 1 – Alexander of Macedonia wins the Battle of Gaugamela (near ancient Ninevah) over the Persian King Darius III. Darius turns his chariot and flees. Alexander chases the Persian forces to Arbela. Darius escapes with his Bactrian cavalry and Greek mercenaries into Media.
- Alexander encounters for the first time war elephants.
- Alexander becomes the master of the Persian Empire. This ends the Achaemenid dynasty. Babylon and Susa open their gates to him. In the capital, Susa, Alexander gets huge treasures amounting to 50,000 gold talents.
- Agis III of Sparta leads some of the Greek cities in a revolt. He holds Crete against Macedonian forces. In the Peloponnesus he routes a force under the Macedonian general Coragus. Athens stays neutral, but he is joined by Elis, Achaea (except Pellene) and Arcadia, with the exception of Megalopolis.
- Alexander of Epirus takes Heraclea from the Lucanians, and Terina and Sipontum from the Bruttii.
- Tarentum turn against Alexander of Epirus when they realize that he wants to make a kingdom of his own in southern Italy. Alexander is defeated and killed in the Battle of Pandosia on the banks of the Acheron.
- The Gallic tribe of the Senones and the Romans enter a period of friendly relations which lasts the rest of the century.