This chapter argues that Marx proffers important resources to develop a critical constitutional theory that questions the fetishism and mystifications of modern constitutionalism and whose goal is to deepen democracy. In particular, it argues that Marx’s critical theory of constitutional law is political, social, socialist, and anti-systemic. The chapter is organised as follows. The first section highlights Marx’s attention to the antinomy of constitutionalism in his engagement with Hegel’s theory of the state. In the course of this engagement, Marx contrasts the political constitution (both monarchical and republican) with ‘democracy’ in order to foreground the constitution’s limitations and point the way toward the resolution of the para...