Zwammerdam is a town in the Dutch province of South Holland along Oude Rijn river. It is a part of the municipality of Alphen aan den Rijn, and lies about 6 km southeast of Alphen aan de Rijn. The name derives from a dam build in the Rhine river in 1165 AD by Count Floris III of Holland, to protect the land stream downwards from floods.
In 2020, the town of Zwammerdam had 1850 inhabitants. The built-up area of the town was 62 km², and contained 484 residences.
The statistical area "Zwammerdam", which also can include the surrounding countryside, has a population of around 1810.
Zwammerdam was a separate municipality until 1964, when it was divided between Alphen aan den Rijn and Bodegraven. The town is the current home of the Jostiband Orchestra.
The ancient Romans did build fortress Nigrum Pullum as part of the defence system of the Roman Empire northern frontier close to what now Zwammerdam is. In the 1970's during groundwork for the building of a new caregiving residence, rests were found of wooden ships from the Roman period.