Leiden University is a public research university in the city of Leiden, in the western Netherlands, with a second campus in the nearby city of The Hague. It is organised into seven faculties covering archaeology, the humanities, law, medicine, science, the social and behavioural sciences, and governance and global affairs, and it teaches roughly fifty bachelor's programmes and more than a hundred master's programmes. Its academic medical activities are carried out through the Leiden University Medical Center.

Established on 8 February 1575, Leiden is the oldest university in the Netherlands. It was founded during the Dutch Revolt and is traditionally associated with William, Prince of Orange; its Latin motto, Libertatis Praesidium ("Bastion of Freedom"), reflects that origin. The university enrolled about 33,000 students as of 2024 and employs several thousand academic and support staff across its two host cities.

Leiden has a long record of achievement in the natural sciences and the humanities. The physicist Pieter Zeeman described the Zeeman effect there in 1896, and Heike Kamerlingh Onnes first liquefied helium in his Leiden laboratory in 1908, work that led on to his discovery of superconductivity. The institution counts sixteen Nobel laureates among its scholars and alumni, and its former students include several Dutch monarchs and prime ministers as well as the American president John Quincy Adams. Leiden is a founding member of the League of European Research Universities and also belongs to the Coimbra Group and the Europaeum. It remains one of the country's most research-intensive universities, active across a broad range of disciplines.