This study considers Chief Justice John Marshall’s famous opinion in Marbury v. Madison (1803) as a vehicle for investigating contemporary interpretations of both John Marshall and the concept of constitutional legitimacy. In it, I examine how Marshall’s opinion located legitimacy in several aspects of the Constitution, including its protection of rights, its embodiment of the consent of the governed, and its ability to organize and direct national politics. Thus, I suggest that Marshall offers a more comprehensive theory of constitutional legitimacy than many recent conceptualizations. Yet more than simply uniting existing approaches to constitutional legitimacy, I demonstrate that Marbury offers a unique theory of the Constitution’s moral legitimacy as well. This analysis of Marbury invites a new appraisal of Marshall as not only a legal and political thinker, but also a constitutional theorist with a distinctive understanding of the American Constitution and its role in the early years of the republic.