|Millennium:||1st millennium BC|
|225 BC by topic|
|Gregorian calendar||225 BC|
|Ab urbe condita||529|
|Ancient Egypt era||XXXIII dynasty, 99|
|- Pharaoh||Ptolemy III Euergetes, 22|
|Ancient Greek era||138th Olympiad, year 4|
|Balinese saka calendar||N/A|
|Chinese calendar||乙亥年 (Wood Pig)|
2472 or 2412
— to —
丙子年 (Fire Rat)
2473 or 2413
|Coptic calendar||−508 – −507|
|Ethiopian calendar||−232 – −231|
|- Vikram Samvat||−168 – −167|
|- Shaka Samvat||N/A|
|- Kali Yuga||2876–2877|
|Iranian calendar||846 BP – 845 BP|
|Islamic calendar||872 BH – 871 BH|
|Minguo calendar||2136 before ROC|
|Seleucid era||87/88 AG|
|Thai solar calendar||318–319|
−98 or −479 or −1251
— to —
−97 or −478 or −1250
Year 225 BC was a year of the pre-Julian Roman calendar. At the time it was known as the Year of the Consulship of Papus and Regulus (or, less frequently, year 529 Ab urbe condita). The denomination 225 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.
- A coalition of Cisalpine Gallic tribes (Taurini, Taurisces, Insubres, Lingones, Salasses, Agones, and Boii), reinforced by large numbers of Transalpine adventurers called Gaesatae (Gaesati), invade Italy. Avoiding the Romans at Ariminum, the Gauls cross the Apennines into Etruria, plunder the country.
- To meet this invasion, the Romans call on the Insubres' enemies, the Adriatic Veneti, the Patavini, and the Cenomani, who rapidly mobilise defensive forces. These armies are placed under the command of consuls Lucius Aemilius Papus and Gaius Atilius Regulus. After the Battle of Faesulae (near Montepulciano) between the Gauls and a Roman army in which the Romans lose many men, the combined Roman forces succeed in outmaneuvering the Gauls and force the invaders towards the coast of Tuscany.
- Seleucus III Ceraunus succeeds his father Seleucus II Callinicus as ruler of the Seleucid dynasty, and takes up the task of reconquering Pergamum in Anatolia from Attalus. However, Andromachus, the first general whom he sends, is decisively defeated and captured by Attalus.