|Millennium:||1st millennium BC|
|Centuries:||5th century BC – 4th century BC – 3rd century BC|
|Decades:||330s BC 320s BC 310s BC – 300s BC – 290s BC 280s BC 270s BC|
|Years:||312 BC 311 BC 310 BC – 309 BC – 308 BC 307 BC 306 BC|
|309 BC by topic|
|Gregorian calendar||309 BC|
|Ab urbe condita||445|
|Ancient Egypt era||XXXIII dynasty, 15|
|- Pharaoh||Ptolemy I Soter, 15|
|Ancient Greek era||117th Olympiad, year 4|
|Chinese calendar||辛亥年 (Metal Pig)
2388 or 2328
— to —
壬子年 (Water Rat)
2389 or 2329
|Coptic calendar||−592 – −591|
|Ethiopian calendar||−316 – −315|
|- Vikram Samvat||−252 – −251|
|- Shaka Samvat||N/A|
|- Kali Yuga||2793–2794|
|Iranian calendar||930 BP – 929 BP|
|Islamic calendar||959 BH – 958 BH|
|Minguo calendar||2220 before ROC
|Seleucid era||3/4 AG|
|Thai solar calendar||234–235|
|Wikimedia Commons has media related to 309 BC.|
Year 309 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Dictatorship of Cursor (or, less frequently, year 445 Ab urbe condita). The denomination 309 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 personally commands a fleet that captures the coastal regions of Lycia and Caria from Antigonus.
- Cassander, who has held Roxana, widow of Alexander the Great, in prison for a number of years, has her put to death along with her young son Alexander, the nominal King Alexander IV of Macedon.
- Antigonus attempts to renew his alliance with the Macedonian general and former regent Polyperchon, who still controls part of the Peloponnesus. He sends Heracles, the illegitimate son of Alexander the Great, to Polyperchon to be treated as a pretender to the throne of Macedonia.
- Polyperchon manages to form an army consisting of 20,000 infantry and 1,000 cavalry and challenges Cassander's army. Instead of fighting, Cassander starts negotiations with Polyperchon. By offering to make him a general of his own army and placing him as governor of Peloponnesus, he convinces Polyperchon to change allegiance to him instead of Heracles. As a result Polyperchon murders Heracles and his mother Barsine.
- Areus I succeeds his grandfather Cleomenes II as king of Sparta.
- A census is carried out in Athens. 21,000 citizens, 10,000 foreign residents and 400,000 others – women, children and slaves – are living in the city.
- Since 480 BC, an aristocratic Council of Elders has effectively ruled Carthage. The titular king of Carthage, Bomilcar, attempts a coup to restore the monarchy to full power. His attempt fails, which leads to Carthage becoming in name as well as in fact a republic.
- Leaving his brother Antander to continue the defence of Syracuse, Agathocles lands in North Africa with the aim of distracting the Carthaginians from their siege of Syracuse. Agathocles concludes a treaty with Ophellas, ruler of Cyrenaica. He then takes advantage of the civil unrest in Carthage and nearly succeeds in conquering the city.
- The Samnites again rise against Rome. Lucius Papirius Cursor is appointed dictator for the second time and wins a great victory at Longula over the Samnites.
- Soon after the State of Qin has conquered the State of Shu (in modern-day Sichuan province), they employ the Shu engineer Bi Ling to create the Guanxian irrigation system, which will eventually provide for over five million people in an area of 40 to 50 square miles (130 km2), still in use today.
- Roxana, wife of Alexander the Great, and mother of Alexander IV of Macedon
- King Alexander IV of Macedon (b. 323 BC)
- Heracles, illegitimate son of Alexander the Great and claimant to the throne of Macedon (b. 327 BC)
- Cleomenes II, King of Sparta
- Zhang Yi, strategist of the Chinese state of Qin