In April 2017, Katwijk had a population of 64.589 and covers an area of 31.13 km2 (12.02 sq mi), of which 6.59 km2 (2.54 sq mi) is water.
Katwijk is by far the largest town in the Duin- en Bollenstreek ("Dune and Bulb Region").
The town consists of a number of districts, including namesakes Katwijk aan den Rijn and Katwijk aan Zee. On 1 January 2005 the various districts had the following populations:
Lying on the coast, Katwijk aan Zee is (and has always been) the larger town. Katwijk aan den Rijn lies just slightly inland. Rijnsburg is situated east of Katwijk aan den Rijn whereas Valkenburg finds itself to the south. Katwijk-Noord is situated north of Katwijk aan Zee.
Although consisting of historically separate towns (except Katwijk-Noord) that still today maintain separate identities, the towns have more or less grown together and merged into a single conurbation. The creation of the municipality of Katwijk is a recognition of that fact. The town hall of the current municipality of Katwijk is located in Katwijk aan den Rijn, near the boundary of Katwijk aan Zee. All these districts lie along the Oude Rijn.