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 I Soter personally commands a fleet that takes the coastal regions of Lycia and Caria from Antigonus I Monophthalmus, capturing the cities of Phaselis, Xanthos, Kaunos, Iasos and Myndus.^[1]

• Lysimachus, the Macedonian ruler of Thrace and one of diadochi, founds the city of Lysimachia on the north-western extremity of the Thracian Chersonese (the modern Gallipoli peninsula) to serve as his capital.^[2]

• 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.^[2]
• 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.
• Ptolemy arrives at Kos and sends for Ptolemaeus (the nephew of Antigonus – who has revolted against Antigonus) to join him to discuss strategy. After finding out Ptolemaeus plans to take over his army and fleet, Ptolemy forces him to commit suicide.^[1]

• 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.

• Ptolemy II Philadelphus, King of Egypt (d. 246 BC)

• King Alexander IV of Macedon (b. 323 BC)
• Cleomenes II, Agiad King of Sparta
• Heracles, illegitimate son of Alexander the Great and claimant to the throne of Macedon (b. 327 BC)
• Ptolemy (general) general of Antigonus I Monophthalmus
• Roxana, wife of Alexander the Great, and mother of Alexander IV of Macedon
• Zhang Yi, strategist of the Chinese state of Qin