Johannes Antonius Marius "Jan" Klabbers (born 13 August 1963, Heumen, Netherlands) is a Dutch Academy Professor (Martti Ahtisaari Chair) at the University of Helsinki, on leave from his regular position as Professor of International Law at the University of Helsinki.
He has previously held several positions at the University of Amsterdam, where he also completed his doctoral degree.
Klabbers is considered to be one of the world’s leading experts in the law of treaties and the law of international organizations.
He has published several monographs and articles on the topics, some of the most important ones being The Concept of Treaty in International Law (1996), Treaty Conflict and the European Union (2008), An Introduction to International Organizations Law 3rd ed.
(2015, previously An Introduction to International Institutional Law), and International Law (2013).