The Kagerplassen (a Dutch term meaning "the Kaag Lakes") is a small lake system in South Holland located to the northeast of Leiden. The Kaag Lakes are a popular area for boating, watersports, fishing, camping and walking. Windmills, waterfront pasture land (weilanden) with grazing animals, quaint Dutch boats and buildings and (in season) flower fields are all part of the charm of boating and walking in this area.

The Kaag Lakes are part of two municipalities: Kaag en Braassem and Teylingen. Three villages lie directly on the lakes: Warmond, De Kaag and Buitenkaag. A number of other villages and towns (including Leiden itself) are located on connecting waterways providing easy access to the lakes.

The Kaag Lakes consist of an interconnected system of smaller lakes and waterways. The Kaag Lakes are fenland lakes (veenplassen) and part of the Holland-Utrecht wet fenland area (veenweidegebied). There are several islands, including the following:

The lakes are home to several sailing schools. The oldest sailing school is 't Vossenhol, founded in 1947 and situated on the Dieperpoel. In 1952 sailing school De Kaag was founded.

The Kaag Lakes are connected to a larger network of lakes and waterways called the Holland Lakes (Hollandse Plassen). The other lake systems are called the Braassemermeer, Wijde Aa and the Westeinderplassen. They are all connected by the Haarlemmermeer Ring Waterway, the Does river and many smaller waterways. This network means that much of northern South Holland is enjoyable for boating.