Year 231 BC was a year of the pre-Julian Roman calendar. At the time it was known as the Year of the Consulship of Matho and Maso (or, less frequently, year 523 Ab urbe condita). The denomination 231 BC for this year has been used since the early medieval period, when the Anno Dominicalendar era became the prevalent method in Europe for naming years.
Demetrius II, king of Macedon, seeks military help from Agron, King of Illyria, a loosely organized state on the Adriatic coast north of Epirus, against the advancing Aetolians. The Illyrian army routs the Aetolians and returns home as victors.