Katwijk aan den Rijn is a population centre in the municipality of Katwijk in the province of South Holland, in The Netherlands, with approximately 6020 inhabitants. Katwijk aan den Rijn lies between Katwijk aan Zee, Valkenburg and Rijnsburg. It actually forms a unit with all of these other centres.
Katwijk aan den Rijn is located near the mouth of the Oude Rijn. The village's name first appeared in the official documents in 1231. At that time, Floris IV, Count of Holland, had his residence there. The village itself had been established about a century earlier. In 1388 the village lost its fish market to the growing village of Katwijk aan Zee. The two places then were within the same seigniory (heerlijkheid). In the early days, Katwijk aan Zee was a fishing village while Katwijk aan den Rijn was more agricultural. Initially the two villages were separate, but over the course of time, they grew and eventually merged.