|Millennium:||1st millennium BC|
|240 BC by topic|
|Gregorian calendar||240 BC|
|Ab urbe condita||514|
|Ancient Egypt era||XXXIII dynasty, 84|
|- Pharaoh||Ptolemy III Euergetes, 7|
|Ancient Greek era||135th Olympiad (victor)¹|
|Balinese saka calendar||N/A|
|Chinese calendar||庚申年 (Metal Monkey)|
2457 or 2397
— to —
辛酉年 (Metal Rooster)
2458 or 2398
|Coptic calendar||−523 – −522|
|Ethiopian calendar||−247 – −246|
|- Vikram Samvat||−183 – −182|
|- Shaka Samvat||N/A|
|- Kali Yuga||2861–2862|
|Iranian calendar||861 BP – 860 BP|
|Islamic calendar||887 BH – 886 BH|
|Minguo calendar||2151 before ROC|
|Seleucid era||72/73 AG|
|Thai solar calendar||303–304|
−113 or −494 or −1266
— to —
−112 or −493 or −1265
Year 240 BC was a year of the pre-Julian Roman calendar. At the time it was known as the Year of the Consulship of Centho and Tuditanus (or, less frequently, year 514 Ab urbe condita). The denomination 240 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.
- Two of Carthage's mercenary commanders – Spendius and Mathos – convince the Libyan conscripts in the mercenary army, that is currently occupying the Carthaginian city of Tunis, to accept their leadership. They persuade the native Libyans that Carthage will take revenge against them for their part in the conflict once the foreign mercenaries are paid and sent home. They then convince the combined mercenary armies to revolt against Carthage and convince the various native Libyan towns and cities to back the revolt. Spendius and Mathos then take the Carthaginian commander Gesco as a hostage. What has started as an argument over pay owed to soldiers by the Carthaginian Government, explodes into a full-scale revolt, known as the Mercenary War.
- The Libyan forces loyal to the mercenaries besiege the towns of Utica and Hippacritae, which refuse to defect to the mercenaries.
- Hanno the Great is given command of the Carthaginian forces. However, the mercenaries defeat the Carthaginian armies in the Battle of Utica.
- Carthage decides to give Hamilcar Barca joint command with Hanno the Great. Hamilcar Barca is able to end the siege of Utica by the mercenaries. He is then placed in complete command of the Carthaginian forces and defeats the mercenaries in the Battle of the Bagradas River.
- After the Numidian mercenary leader Narawas defects to Hamilcar Barca, Numidian reinforcements (about 2,000 men) help him defeat the mercenaries again. Hamilcar pardons his captured prisoners, accepting into his army anyone who will fight for Carthage, and exiling anyone who will not.
- May 25 – Chinese astronomers make the first recorded observation of Halley's Comet.
- The Hellenistic period Mathematician Eratosthenes estimated the Earth's circumference to be 252,000 stadia, a figure between 2%-20% off modern measurements.
- Aratus, Greek poet from Soli in Cilicia, best remembered for his poem on astronomy Phaenomena (b. c. 315 BC)
- Aristomachos the Elder, Greek tyrant from Argos
- Asandhimitra, Mauryan empress and wife of Ashoka
- Callimachus, Greek poet and librarian
- Posidippus, Greek epigrammatic poet
- Zou Yan, Chinese philosopher (b. 305 BC)