In customary international law, opinio juris is the second element (along with state practice) necessary to establish a legally binding custom. Opinio juris denotes a subjective obligation, a sense on behalf of a state that it is bound to the law in question. See ICJ Statute, Article 38(1)(b) (the custom to be applied must be "accepted as law").
Whether the practice of a state is due to a belief that it is legally obliged to do a particular act is difficult to prove objectively. Therefore, opinio juris is an unsettled and debated notion in international law.
Although commonly referenced as simply opinio juris, the term is derived from the Latin phrase opinio juris sive necessitatis ("an opinion of law or necessity").
- See North Sea Continental Shelf Cases (West Germany v. Neth./Denmark) (1968-69) (holding that states' frequent or habitual performance of certain actions does not, by itself, establish opinio juris).