John Marshall’s Constitutionalism is an exploration of Supreme Court Chief Justice John Marshall’s political thought. Often celebrated and occasionally derided as a force in the creation of American jurisprudence and the elevation of the American Supreme Court, Marshall is too seldom studied as a political thinker. Clyde H. Ray explores this neglected dimension of Marshall’s thought by examining his constitutional theory in the context of several of his most important Supreme Court opinions, arguing that Marshall’s political theory emphasized the federal Constitution’s fundamental legitimacy; its sovereignty over national and state government policy; its importance in defining responsible citizenship; and its role in establishing a Constitution-based form of American nationalism. This cross-disciplinary argument illustrates Marshall’s devotion to the Constitution as a new source of national identity during the early national period. Furthermore, Ray argues that Marshall’s constitutionalism makes important contributions not only to our understanding of American constitutionalism during his time, but also conveys important lessons for readers seeking a better understanding of the Constitution’s role in the United States today.