|Millennium:||1st millennium BC|
|378 BC by topic|
|Gregorian calendar||378 BC
|Ab urbe condita||376|
|Ancient Egypt era||XXX dynasty, 3|
|- Pharaoh||Nectanebo I, 3|
|Ancient Greek era||100th Olympiad, year 3|
|Balinese saka calendar||N/A|
|Chinese calendar||壬寅年 (Water Tiger)
2319 or 2259
— to —
癸卯年 (Water Rabbit)
2320 or 2260
|Coptic calendar||−661 – −660|
|Ethiopian calendar||−385 – −384|
|- Vikram Samvat||−321 – −320|
|- Shaka Samvat||N/A|
|- Kali Yuga||2723–2724|
|Iranian calendar||999 BP – 998 BP|
|Islamic calendar||1030 BH – 1029 BH|
|Minguo calendar||2289 before ROC
|Thai solar calendar||165–166|
−251 or −632 or −1404
— to —
−250 or −631 or −1403
Year 378 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Tribunate of Medullinus, Fidenas, Lanatus, Siculus, Pulvillus and Macerinus (or, less frequently, year 376 Ab urbe condita). The denomination 378 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.
- The Theban general and statesman, Epaminondas, takes command of Thebes. Pelopidas is elected boeotarch, or chief magistrate, of the city.
- Timotheus, the son of the Athenian general Conon, is elected strategos of Athens.
- A Spartan attempt to seize Piraeus brings Athens closer to Thebes. The Athenian mercenary commander Chabrias successfully faced off the larger army of Agesilaus II near Thebes. At the advance of Agesilaus' forces, instead of giving the order to charge, Chabrias famously ordered his men at ease—with the spear remaining pointing upwards instead of towards the enemy, and the shield leaning against the left knee instead of being hoisted against the shoulder. The command was followed immediately and without question by the mercenaries under his command, to be copied by their counterparts beside them, the elite Sacred Band of Thebes under the command of Gorgidas. This "show of contempt" stopped the advancing Spartan forces, and shortly afterwards Agesilaus withdrew.
- Athens allies itself with Thebes and forms the Second Athenian Empire. The confederacy includes most of the Boeotian cities and some of the Ionian islands.
- Dionysius I's third war with Carthage proves disastrous. He suffers a crushing defeat at Cronium and is forced to pay an indemnity of 1,000 talents and cede the territory west of the Halycus River to the Carthaginians.
- The Servian Wall is constructed around Rome to prevent the city from being captured or sacked (see 390 BC). This is the first fortification that the Romans build around their home city.
- Mark H. Munn (1993). The Defense of Attica: The Dema Wall and the Boiotian War of 378-375 B.C. University of California Press. ISBN 978-0520076853.
- An Illustrated Encyclopedia: "The Uniforms of the Roman World", Kevin F. Kiley (2012). Roman Republic Timeline 753–132 BC, p. 14. ISBN 978-0-7548-2387-2