|Millennium:||1st millennium BC|
|312 BC by topic|
|Gregorian calendar||312 BC|
|Ab urbe condita||442|
|Ancient Egypt era||XXXIII dynasty, 12|
|- Pharaoh||Ptolemy I Soter, 12|
|Ancient Greek era||117th Olympiad (victor)¹|
|Balinese saka calendar||N/A|
|Chinese calendar||戊申年 (Earth Monkey)|
2385 or 2325
— to —
己酉年 (Earth Rooster)
2386 or 2326
|Coptic calendar||−595 – −594|
|Ethiopian calendar||−319 – −318|
|- Vikram Samvat||−255 – −254|
|- Shaka Samvat||N/A|
|- Kali Yuga||2789–2790|
|Iranian calendar||933 BP – 932 BP|
|Islamic calendar||962 BH – 961 BH|
|Minguo calendar||2223 before ROC|
|Seleucid era||0/1 AG|
|Thai solar calendar||231–232|
−185 or −566 or −1338
— to —
−184 or −565 or −1337
Year 312 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Consulship of Corvus and Mus (or, less frequently, year 442 Ab urbe condita). The denomination 312 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.
- Ptolemy and Seleucus, the satrap of Babylonia, invade the satrapy of Syria. The resulting Battle of Gaza leads to a triumph for Ptolemy and Seleucus over Antigonus' son, Demetrius Poliorcetes ("sieger of cities"), who is captured but immediately released. Seleucus ceases his service to Ptolemy and returns to his former province, Babylonia. This event takes place on October 1 and becomes the starting point of the Seleucid era.
- Telesphorus (general) enters Elis and fortifies the citadel, and enslaves the city.
- Telesphorus (general) ends his friendship with Antigonus through betrayal
- The Syracusans ask for help against their tyrant Agathocles from the Carthaginians, who, fearing for their own possessions in Sicily, send a large force to the island.
- The Roman censor, Appius Claudius Caecus, a patrician, enters office and begins construction of the Appian Way (the Via Appia) between Rome and Capua. He also embarks on a program of political reform, including the distribution of the landless citizens of Rome among the tribes, which at this time constitute basic political units. Appius also admits sons of freedmen into the Roman Senate. He also asserts the right of freed slaves to hold office.
- Rome gets its first pure drinking water as engineers complete the first aqueduct into the city, the Aqua Appia.