In the framework of the contemporary discussion about the crisis of the notion of the law, this article presents an introduction to Jean-Paul Sartre's critical delimitation to the concept of the law. For that reason, Sartre's ontological, theoretical, and dialectic foundations are outlined in order to comprehend the Sartrean conception of the law, highlighting the emancipatory potential of the law found in popular justice. It will be argued that Sartre's definition of popular justice as a legal form to control political power rests on the basis of the definition of the law as a negation of the self, the detotalization of man, and the oppressive institution of the dominant class.