opinio juris (international law)

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Opinio juris is a shortened form of the Latin phrase “opinio juris sive necessitatis,” which means "an opinion of law or necessity."  

In customary international law, opinio juris is the second element necessary to establish a legally binding custom. Opinio juris denotes a subjective obligation, in that a state perceives itself to be bound by the law in question. The International Court of Justice outlined the standard for customary international law in ICJ Statute, Article 38(1)(b) which requires that the custom be “a general practice accepted as law.” The general practice requirement refers to state practice or actions, while the accepted as law aspect refers to opinio juris, the belief that the actions are required by law. 

Practices that are generally followed by states but which they feel free to legally ignore lack the opinio juris element and thus are not customary international law. At the same time, a practice that was originally to be followed for other reasons may become customary international law if states come to believe they have a legal obligation to the practice.

Opinio juris can be inferred by the context and manner in which states comply with customary international law. As with customary international law, opinio juris is an unsettled and debated concept in international law.

[Last updated in July of 2023 by the Wex Definitions Team]