This article considers Baruch Spinoza's contribution to a theory of constituent power. Modern theories of constituent power generally agree on its paradoxical essence: a power that comes before the law and founds the law is at the same time a power that, once the juridical sphere is established, has to be obliterated by the law. Spinoza's ontology has been recognised as one of the early modern sources of constituent power, yet he argues for a strict equivalence between law and power. This article argues that by reading Spinoza's political theory through the lens of a radical immanence between ontology and history, we can understand him as a source for a theory of constituent power. It also argues that, through this immanence, Spinoza's thought offers a solution to the paradox of constituent power and enriches contemporary discussions on the origin of juridical sphere and the relationship between politics and law. Since the foundation of early modern sovereignty, juridical science has tackled the problem of establishing a solid ground for authority. However, far from finding a stable solution, the problem has become more serious during the centuries of bourgeois revolutions, when new forms of power took the place of older ones and the search for a new foundation began in the nature and attributes of the new revolutionary subjects. This problem has gradually assumed a definite legal form and has taken the name of 'constituent power'. By constituent power, modern juridical science means the factual and political power that establishes a new legal order, assigning to it validity and efficacy. The constituent power is therefore pre-ordered and super-ordered vis-à-vis the constituted power: it founds and precedes the establishment of law.Modern scholars who, especially after World War II, have developed this concept, highlight the ambiguous (Böckenförde, 1991), extreme (Schmitt, 2008) and seemingly contradictory (Mortati, 1972) character of constituent power.