The purpose of this article is to analyse the legality of the practice of treaty-based interventions by international organisations of regional character in their member states under international law. In particular, the objective is to analyse the legality of this practice in relation to the general rules of international responsibility and to the collective security system of the Charter of the United Nations. Regarding the rules of international responsibility treaty-based interventions are considered lawful provided there is valid consent from the affected state, since consent is considered as a circumstance precluding the wrongfulness of certain conducts. With respect to the United Nations Charter such practice can also be deemed lawful considering that it does not characterize as enforcement action requiring authorization by the Security Council, as provided for in Chapter VIII of the Charter. The article concludes that such interventions are generally lawful under international law but must comply with certain conditions to be carried out. As a recent practice, interventions by regional organisations in their member states do not have well-defined legal dimensions and have not been extensively analysed by doctrine.