Leiderdorp is a town and municipality in the western Netherlands, in the province of South Holland near the city of Leiden. It had a population of 26,182 in 2004. The municipality covers an area of 12.29 km² (4.75 mile²) of which 0.56 km² (0.22 mile²) is water. Leiderdorp has now become a suburb of the city of Leiden, although the Oude Rijn (Old Rhine) river and the Zijl river separate the two. The HSL-Zuid high-speed rail line between Amsterdam and Brussels crosses Leiderdorp in a tunnel. It is one of the oldest towns in Zuid Holland province and was a base for the Spanish army in the Eighty Years' War. Šamorín, in Slovakia, is the twin town of Leiderdorp.