This article argues that Spinoza is a modern republican political philosopher. The first section of the paper discusses the influence of natural law and social contract theories on Spinoza’s view. The second section discusses the problem of state formation and shows that Spinoza takes collective action problems seriously. He solves the problem as it applies to less than rational individuals through the institution of civil religion. The last sections of the paper develop and defend the view that Spinoza is a republican theorist. This claim has four basic elements. First, the state depends on a continuous and dynamic transfer of power from its citizens, which is defined as participation in public life. Second, the stability of a state depends on how effectively the regime can foster participation in the state. Hence the republican ideal of government is expressed not so much in any particular constitutional form of the state but in how well each form can foster participation. The state’s relation to its citizens is defined through the minimal and maximal forms of participation. The minimal definition is simply non-interference in the activities of others; the maximal definition is the promotion of a common good. Third, the participation of the individual in the state is not an end in itself but the means to the individual’s own freedom. So, although participation in the state is a necessary condition of individual well-being, it is certainly not sufficient. Fourth, the participation of individuals in the state, the quality and structure of state stability, as a well as the freedom of the state and individual, all depend on the degree of rationality manifest in both the individual and in the institutional structures of the state. In other words, Spinoza’s theory of knowledge and the passions have an important role to play in his political theory. To the extent that individuals act on the basis of reason and can check their irrational passions, and to the extent that the institutions of the state are developed in accordance with a scientific understanding of human nature, the state will be more stable and the individual more free.