This work tries to understand the legal nature of ius gentium as from the thought of St. Thomas. While most authors have strongly emphasized its positive character, remaining blindly faithful to the express words of saint Thomas in I-II, 94, 5, they have recklessly left unattended the words St. Thomas dedicates to this subject in his Commentary on the Nicomachean Ethics and in his Treatise of Justice. The novelty of our interpretation lies precisely in a harmonious reading of all the texts in which Aquinas deals in some depth with the problem of the legal nature of the norms of ius gentium. It is thereby our believe that, despite his assertions on the Treatise of Law, where he presented ius gentium as being part of what is to be held as positive law, the real thesis of saint Thomas is rather the affirmation of its natural (non-positive) character.