|Millennium:||1st millennium BC|
|109 BC by topic|
Year 109 BC was a year of the pre-Julian Roman calendar. At the time it was known as the Year of the Consulship of Numidicus and Silanus (or, less frequently, year 645 Ab urbe condita). The denomination 109 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.
- Han campaigns against Dian: Emperor Wu of Han launches a new campaign against the Dian Kingdom and establishes the Yizhou commandery in Yunnan during the dynasty's expansion southward.
- A Roman army under Marcus Junius Silanus (consul 109 BC) is defeated by the Cimbri and Teutones near the river Rhone.