The site of the town has been occupied since prehistoric times and is scattered with Roman remains. It became a medieval market town, after Keynsham Abbey was founded around 1170. It is situated at the confluence of the River Chew and River Avon and was subject to serious flooding before the creation of Chew Valley Lake and river level controls at Keynsham Lock. The Great Flood of 1968 inundated large parts of the town. It was home to the Cadbury's chocolate factory, Somerdale, which opened in 1935 as a major employer in the town.