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Opinio juris sive necessitatis or Opinio juris (loosely translated into English as "opinion of justice") is the belief that a behavior was done because it was a legal obligation. This is in contrast to a behavior being the result of different cognitive reaction, or behaviors that were habitual to the individual. This term is frequently used in legal proceedings such as a defense for a case.

Opinio juris is the subjective element of customary international law as it refers to beliefs. The other element is state practice, which is more objective as it is readily discernable. To qualify as state practice, the acts must be consistent and general international practice.

A situation where Opinio juris would be feasible is a case concerning Self-defense. A condition must be met where the usage of force is limited to the situation at hand. The act of striking an attacker may be done with legal justification, however legal territory limits the acceptability of such a claim. Even in this case, the usage of force must be acceptable to the conditions of the environment, the attacker, and the physical conditions of the people involved, as well as any weapons or tools used.

In international law, opinio juris is the subjective element which is used to judge whether the practice of a state is due to a belief that it is legally obliged to do a particular act. It can sometimes be difficult to establish opinio juris, but where there is consistent practice over a length of time, the need for opinio juris is lessened. Where there is more sporadic state practice, the presence of opinio juris becomes more important. In addition, the existence of custom in general need not be world-wide, but can also be restrained to the region. Customary international law has been deemed a source of international law under Article 38(1)(b) of the Statute of the International Court of Justice.