Ratification is a principal's approval of an act of its agent that lacked the authority to bind the principal legally. Ratification defines the international act in which a state indicates its consent to be bound to a treaty if the parties intended to show their consent by such an act. In the case of bilateral treaties, ratification is usually accomplished by exchanging the requisite instruments, and in the case of multilateral treaties, the usual procedure is for the depositary to collect the ratifications of all states, keeping all parties informed of the situation.

The institution of ratification grants states the necessary time-frame to seek the required approval for the treaty on the domestic level and to enact the necessary legislation to give domestic effect to that treaty. The term applies to private contract law, international treaties, and constitutions in federal states such as the United States and Canada. The term is also used in parliamentary procedure in deliberative assemblies.