Martti Antero Koskenniemi (born 18 March 1953, Turku) is an international lawyer and a former Finnish diplomat. Currently he is professor of International Law in the University of Helsinki and Director of the Erik Castrén Institute of International Law and Human Rights, as well as Centennial Professor at the Law Department of the London School of Economics. He is well known for his critical approach to international law. In 2008–2009 he held the seat of distinguished visiting Goodhart Professor at the Faculty of Law, Cambridge University. In 2011 Koskenniemi was Peace of Utrecht professor at Utrecht University. In 2014 he was elected a Corresponding Fellow of the British Academy. Koskenniemi is currently serving as an Academy Professor for the Academy of Finland.
Previously he has been Global Professor of Law in the New York University, and a member of the International Law Commission (2002–2006). He served in the Finnish Diplomatic Service in the years 1978-1996, lastly as director of the Division of International Law. He was Finland's counsel in the International Court of Justice in the Passage through the Great Belt case (Finland v. Denmark) (1991–1992)
From 1997 to 2003 he served as a judge in the administrative tribunal of the Asian Development Bank.
He is a member of the Institut de droit international.
From Apology to Utopia; The Structure of International Legal Argument (first published 1989) presents a critical view of international law as an argumentative practice that attempts to remove the political from international relations. It asserts that international law is vulnerable to criticisms of being either an irrelevant moralist utopia or an apology to Realpolitik.
The Gentle Civilizer of Nations: The Rise and Fall of International Law 1870-1960 (2001) has two agendas, first of which is to develop an intellectual history of international law, and to offer a critique of that history. The second is to offer a sociology of the profession of international law, using biographical studies of Hersch Lauterpacht, Carl Schmitt and Hans Morgenthau.