|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|
|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|
|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||2771–2772|
|Iranian calendar||952 BP – 951 BP|
|Islamic calendar||981 BH – 980 BH|
|Minguo calendar||2242 before ROC
|Thai solar calendar||212–213|
|Wikimedia Commons has media related to 331 BC.|
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.
- Alexander departs from Egypt and leads his forces towards Phoenicia. He leaves Cleomenes of Naucratis as the ruling nomarch to control Egypt.
- October 1 – Alexander of Macedonia is victorious in the Battle of Gaugamela (near ancient Ninevah) over the Persian King Darius III. Darius turns his chariot and flees, although his subordinates fight on. Alexander pursues the defeated Persian forces to Arbela, but Darius escapes with his Bactrian cavalry and Greek mercenaries into Media.
- Alexander encounters for the first time war elephants, he captures 15 Persian elephants after the battle in Darius' camp.
- Alexander becomes the master of the Persian Empire, ending the Achaemenid dynasty. Babylon and Susa open their gates to him. 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 routes 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 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.
- Alexander I of Epirus, Aeacid dynasty king of Epirus (b. c. 370 BC)
- Vahe, legendary king of Armenia and last of the Hyke dynasty