In a series of recent opinions, the Supreme Court has threatened to transform the nondelegation doctrine into a device for imposing sweeping limits on congressional authority to empower the regulatory state. But, as a matter of history and logic, the nondelegation doctrine has a quite different purpose. This Article argues that the nondelegation doctrine plays an underappreciated role in constitutional structure: encouraging the segmentation of executive power. The nondelegation doctrine vindicates the Article I Vesting Clause by preventing Congress from being divested of its legislative power. Its purpose is to reinforce Congress’s legislative supremacy in the realm of ordinary law, not to impede Congress’s ability to achieve legislative objectives by delegating regulatory authority to administrative agencies. The nondelegation doctrine accomplishes its distinctly structural purpose by constraining the delegation of broad powers to the President directly, a constraint that encourages legislative delegation of regulatory authority to administrative agencies. The Article explains as a matter of theory why broad delegations to the President, unlike the delegation of substantial regulatory authority to administrative agencies, jeopardize legislative supremacy and hence pose heightened nondelegation concerns, and it finds strong support for this distinction in the history of nondelegation decisions. It concludes that the diffuse departmental structure of the modern administrative state is a testament to the great success of the nondelegation doctrine, not evidence of its underenforcement. Indeed, the contemporary push to reinvent the nondelegation doctrine in an indiscriminate way would turn it into something closer to its opposite, a cudgel against legislative supremacy rather than its guardian.