Ter Aar is a town and former municipality in the western Netherlands, in the province of South Holland. It is located about 6 km northeast of Alphen aan den Rijn. The town had a population of 9,007 on 1 July 2006.

Nieuwkoop is a town and municipality in the western Netherlands, in the province of South Holland. The municipality was enlarged on 1 January 2007, through the amalgamation of Liemeer and Ter Aar. The municipality now covers an area of 91.16 km2 (35.20 sq mi) of which 13.11 km2 (5.06 sq mi) is water. Its population was 29,151 in 2021.