Modern jurisprudence has typically been presented as a debate between legal positivism and natural law. Though the demise of legal positivism has been touted despite its pre-eminence in past decades, it is clear that there remains a vigorous debate surrounding this theory. It is noteworthy that Hans J. Morgenthau’s legal thought and critique of legal positivism have remained unexplored in the context of this debate. Largely forgotten, his legal thought answers questions that lie at the heart of the natural law and legal positivist debate. It showcases his deeply nuanced understanding of legal and political theory and contains a powerful and insightful commentary on the fundamental problems faced by international law. Building on existing literature, this paper unearths Morgenthau’s critique of legal positivism. It does this by re-examining his works, which address the question of whether moral considerations are relevant to determining the content of the law in force. It brings his legal thought to light, which highlights the artificiality of the division between law and morality and offers a nuanced analysis of problems inherent in international law. Ultimately, the paper challenges the claim that the law can be determined without resorting to moral judgement and shows how Morgenthau’s insights remain relevant to legal positivism and natural law debates today.