Duhaime's Law Dictionary

Opinio Juris Definition:

Latin: the sense of legal obligation. In international law, acceptance of a practice as sufficient to create legal obligations.

Related Terms: Customary International Law, Law of Nations

In a 1998 article, Nicholas Onuf wrote, in volume 4 of the law journal International Legal Theory, of:

"... Oppenheim's elegant definition of opinio juris as state practice under the aegis of conviction that the practice is according to international law, obligatory or right or ... the Restatement of Foreign Relations definition as a sense of legal obligation."

Goldsmith and Posner write of opinio juris:

"Opinio juris is described as the psychological component of customary international law because it refers to an attitude that states have toward a behavioral regularity. The idea of opinio juris is mysterious because the legal obligation is created by a state's belief in the existence of the legal obligation. Opinio juris is really a conclusion about a practice's status as international law; it does not explain how a widespread and uniform practice becomes law."

Generally, to create legal obligations, opinio juris, the conviction that a custom is mandatory, must be accompanied by a history or putting it into practice. Lowe gives the somewhat anecdotal examples of opinio juris as head of state immunity and the rolling out of a red carpet upon arrival upon another state's territory.

A strong legal basis for opinio juris is at §38.1(b) of the ICJ Statute (emphasis added):

"The Court, whose function is to decide in accordance with international law such disputes as are submitted to it, shall apply: (a). international conventions, whether general or particular, establishing rules expressly recognized by the contesting states; (b) international custom, as evidence of a general practice accepted as law; (c) the general principles of law recognized by civilized nations; (d) subject to the provisions of Article 59, judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law."


  • Goldsmith, Jack L. and Posner, Eric A., The Limits of International Law (2005), page 24.
  • Loew, Vaughan, International Law (Oxford: University Press, 2007), page 38.
  • Nicar v US, 1986 I.C.J. 4
  • Onuf, Nicholas, 4 Int'l Legal Theory 1 (1998)
  • Schreiber, Ross E., Ascertaining Opinio Juris of States Concerning Norms Involving the Prevention of International Terrorism: A Focus on the U.N. Process, 16 BU ILJ 309 (1998)

Categories & Topics:

Always looking up definitions? Save time with our search provider (modern browsers only)

If you find an error or omission in Duhaime's Law Dictionary, or if you have suggestion for a legal term, we'd love to hear from you!