Kelsen’s legal monism—the claim that it is impossible for legal science to recognize the existence of more than one legal system—is often held to be one of the least plausible aspects of Kelsen’s pure theory of law. This paper challenges the criticism of Kelsen’s monism in the work of H.L.A. Hart. In particular, it will argue that Hart failed to show that Kelsen’s monism rests on a mistaken criterion of the identity of legal system and that it is, therefore, descriptively inadequate. Kelsen’s monism is not only descriptively viable but potentially able to provide an interesting counterpoint to the prevalent legal pluralist orthodoxy in the theory of legal system.