Alphen aan den Rijn is a city and municipality in the western Netherlands, in the province of South Holland, between Leiden and Utrecht. The city is situated on the banks of the river Oude Rijn, where the river Gouwe branches off. The municipality had a population of 110,986 in 2019, and covers an area of 132.50 km² of which 6.27 km² is water.
The municipality of Alphen aan den Rijn also includes the communities of Aarlanderveen, Zwammerdam, and Boskoop. The city is located in what is called the 'Green Heart' of the Netherlands, which is a somewhat less densely populated centre area of the Randstad.
The name "Alphen" is probably derived from the name of the Roman fort Albaniana, meaning "settlement at the white water". Its remains still lie underneath the city centre.