In this article I identify the principles of justice by which an economic union is to be constituted. For this purpose, I extend John Rawls’s constructivist theory of justice to economically integrated societies. With regard to the principles identified, I defend a twofold claim. First, the principles of economic union generated by this extended procedure of construction can serve as common points of reference for the subjects of an economic union. Second, these principles cannot come into conflict with similarly constructed prior principles of social justice and international justice.