Leiden ( pronunciation (help·info) ['lɛi.də(n)]) (in English and archaic Dutch also Leyden) is a city and municipality in the Dutch province of South Holland. The municipality of Leiden has a population of about 120,000, but the city forms one densely connected urban area with its suburbs Oegstgeest, Leiderdorp, Voorschoten, Valkenburg, Rijnsburg and Katwijk, which have about 254,000 inhabitants combined. The larger Leiden agglomeration counts 332,000 inhabitants which makes it the sixth major agglomeration in the Netherlands. Leiden is located on the Old Rhine, at a distance of some 20 kilometers from The Hague to its south and some 40 kilometers from Amsterdam to its north. The recreational area of the Kaag Lakes (Kagerplassen) lies just to the northeast of Leiden.

A university city since 1575, Leiden houses Leiden University and Leiden University Medical Centre. It is twinned with Oxford, the location of England's oldest university.