Alphen aan den Rijn is a city and municipality in the western Netherlands, in the province of South Holland. The city is situated on the banks of the river Oude Rijn, where the river Gouwe branches off. The municipality had a population of 112,587 in 2021, and covers an area of 132.50 km2 (51.16 sq mi) of which 6.27 km2 (2.42 sq mi) is water.