What is opinio iuris?

Opinio iuris is a term used in law that means “conviction of legal binding”. It is used to refer to the subjective or psychological element of custom, which is a source of law that is based on the repetition of conduct by the subjects.

It implies that legal subjects not only carry out a conduct constantly and uniformly, but also consider it as a norm or a legal obligation, and not as a mere convenience.

Opinio iuris is especially applied in international law, where custom is one of the main sources of law. For there to be an international custom, two elements are required:

• State practice, which is the effective and repeated behavior of states.
• Opinio iuris, which is the belief of states that such behavior is mandatory for legal reasons.

For example, there is an international custom that prohibits the use of force between states, based on state practice and the opinio iuris that this is a fundamental legal principle.

Related concepts

• Custom
• Sources of law