This article examines the theoretical confrontation between modernity and postmodernity in legal philosophy and, in particular, analyzes the implications of the postmodern paradigm for legal rationality, normativity, and legitimacy. The main subject of the study is an evaluation of the impact of French post-structuralist thought—especially in the works of Jean-François Lyotard and Jacques Derrida—on the theoretical foundations of modern law. The research adopts a descriptive–analytical method based on documentary analysis. First, the essential components of modern legal thought and the key elements of the postmodern paradigm—including the critique of logocentrism, the concept of deconstruction, the idea of the “death of the subject,” and the end of grand narratives—are outlined. Their consequences for legal interpretation and judicial reasoning are then critically examined. The central question of the article is whether the rejection of foundational rationality and legitimizing grand narratives in postmodernism can provide an alternative basis for legal theory, or whether it ultimately weakens the normative and practical foundations of law. The findings indicate that although postmodernism plays an important critical role in exposing the power-oriented and ideological dimensions of modernity and enables a reconsideration of certain established assumptions in modern legal theory, its abandonment of rational standards of judgment and its relativization of meaning seriously challenge the possibility of distinguishing between legitimacy and arbitrariness. Consequently, the article argues that if postmodernism fully rejects normative rationality, it tends toward relativism or a form of legal nihilism and undermines the capacity to defend responsibility and legal judgment.