Year 331 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Consulship of Potitus and Marcellus (or, less frequently, year 423 Ab urbe condita). The denomination 331 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.
|Millennium:||1st millennium BC|
|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
- Alexander of Macedonia departs from Egypt and leads his forces towards Phoenicia. He leaves Cleomenes of Naucratis as the ruling nomarch to control Egypt.
- October 1 – Alexander is victorious in the Battle of Gaugamela (near ancient Ninevah) over the Persian King Darius III. Alexander pursues the defeated Persian forces to Arbela, Darius moves his Bactrian cavalry and Greek mercenaries into Media.
- Alexander encounters for the first time war elephants after the battle in Darius' camp. In the capital, Susa, Alexander gains access to huge treasures amounting to 50,000 gold talents.
- While Alexander is fighting in Asia, Agis III of Sparta, profiting from the Macedonian king's absence from Greece, leads some of the Greek cities in a revolt. With Persian money and 8,000 Greek mercenaries, he holds Crete against Macedonian forces. In the Peloponnesus he routs a force under the Macedonian general Coragus and, although Athens stays neutral, he is joined by Elis, Achaea (except Pellene) and Arcadia, with the exception of Megalopolis, the staunchly anti-Spartan capital of Arcadia, which Agis III's forces besiege.
- Alexander's regent Antipater leads the Macedonians to victory over King Agis III in the Battle 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 intends to create 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 conclude a peace and enter upon a period of friendly relations which lasts the rest of the century.