|Millennium:||1st millennium BC|
|Centuries:||2nd century BC – 1st century BC – 1st century|
|Decades:||70s BC 60s BC 50s BC – 40s BC – 30s BC 20s BC 10s BC|
|Years:||43 BC 42 BC 41 BC – 40 BC – 39 BC 38 BC 37 BC|
|40 BC by topic|
|Gregorian calendar||40 BC|
|Ab urbe condita||714|
|Bahá'í calendar||−1883 – −1882|
|English Regnal year||N/A|
|Chinese calendar||庚辰年 (Metal Dragon)
2657 or 2597
— to —
辛巳年 (Metal Snake)
2658 or 2598
|Coptic calendar||−323 – −322|
|Ethiopian calendar||−47 – −46|
|- Vikram Samvat||17–18|
|- Shaka Samvat||N/A|
|- Kali Yuga||3062–3063|
|Igbo calendar||−1039 – −1038|
|Iranian calendar||661 BP – 660 BP|
|Islamic calendar||681 BH – 680 BH|
|Julian calendar||40 BC|
|Minguo calendar||1951 before ROC
|Thai solar calendar||504|
|Wikimedia Commons has media related to 40 BC.|
Year 40 BC was either a common year starting on Thursday, Friday or Saturday or a leap year starting on Thursday or Friday (link will display the full calendar) of the Julian calendar (the sources differ, see leap year error for further information) and a common year starting on Friday of the Proleptic Julian calendar. At the time, it was known as the Year of the Consulship of Calvinus and Pollio (or, less frequently, year 714 Ab urbe condita). The denomination 40 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.
- Consuls: Gnaeus Domitius Calvinus and Gaius Asinius Pollio.
- Siege of Perusia: After a last attempt to break the siege, which failed; Lucius Antonius surrenders to Octavian. His life is spared, but the citizens are executed or sold in slavery. Fulvia fled with her children and is exiled to Sicyon, where she died of a sudden illness.
- Spring – Salvidienus Rufus marches to Transalpine Gaul to take command of the eleven legions, after the death of Quintus Fufius Calenus. Octavian divorced Clodia Pulchra and marries Scribonia, a sister of Lucius Scribonius Libo and a follower of Sextus.
- Sextus Pompey dispatched Menas with four legions and captured Sardinia, driving out Octavian's governor Marcus Lurius. He seized the capital, Caralis and occupied Corsica. Sextus besieged Cosenza in Bruttium and Thurii in Lucania, ravaging the territory with his cavalry.
- Sextus' fleet raides the ports of Puteoli and Ostia. The populace hold the Triumvirs responsible for prolonging the war, they provoked a riot on the Forum. Octavian with the Praetorian Guard went to intercept, he escaped with his life because Antony summoned troops to rescue his junior colleague.
- Treaty of Brundisium: The Triumvirs agreed to divide the Roman Republic into spheres of influence. Gaius Octavian styled himself "Imperator Caesar" and takes control of the Western provinces. Mark Antony is given the Eastern provinces; the River Drin, the boundary between the provinces Illyricum and Macedonia, would serve as their frontier. Marcus Aemilius Lepidus controls Hispania and Africa. The treaty is cemented by the marriage of Antony and Octavia, sister of Octavian.
- Quintus Labienus occupied Cilicia and marched with an army into Anatolia. Most cities surrendered without resistance, except for Alabanda and Mylasa. The Parthians restored their territory to nearly the limits of the old Achaemenid Empire, Labienus proclaimed himself "Parthian Emperor" of Asia Minor.
- Mark Antony leaves Alexandria; after receiving news of the outcome at Perusia while en route to Phoenicia, he sets sail for Italy meeting the ambassadors of Sextus Pompey in Athens.
- Athenodorus a philosopher, encounters a ghost in Athens. This popular story is one of the first of the poltergeist stories in history.
- Pacorus crosses with help of Quintus Labienus the Euphrates and invades Syria. The capital Antioch surrenders, the Parthians takes Phoenicia and Judea, the fortified city of Tyre can not besiege because they have no fleet.
- Parthians conquer Jerusalem. Hyrcanus II is removed from power, while Antigonus the Hasmonean becomes king of Judea under Parthian rule. Herod the Great flees Jerusalem to Rome. There he is titled king of Judea by Mark Antony.
- The Ji Jiu Pian dictionary published in this year during the Han Dynasty is the earliest known reference to the hydraulic-powered trip hammer device.
- Cleopatra Selene (d. AD 6) and Alexander Helios (d. between 29 and 25 BC), the twins of Cleopatra VII of Egypt and Mark Antony
- May – Gaius Claudius Marcellus Minor dies, leaving Octavia Minor widowed. She will later marry Mark Antony by decree of the Roman Senate (b. 88 BC)
- Fulvia, wife of Publius Clodius Pulcher and Mark Antony (b. 77 BC)