Alphen aan den Rijn is a city and municipality located in the western part of the Netherlands within the province of South Holland. It is situated on the banks of the river Oude Rijn, which serves as the point where the river Gouwe branches off.
The municipality covers a total area of 132.50 km² (51.16 sq mi), with approximately 6.27 km² (2.42 sq mi) consisting of water. As of 2021, the city had a recorded population of 112,587 residents.