At the request of the Executive Branch, courts routinely dismiss private suits against sitting heads of foreign states. Congress has never delegated authority to the Executive Branch to identify principles governing head of state immunity. The courts’ practice thus appears inconsistent with the conventional view that the Executive Branch lacks authority to affect private rights unless authorized by Congress to do so. This Article argues that the Executive Branch’s practice of determining head of state immunity is an example of sole executive lawmaking, deriving from the President’s constitutional responsibility as the only authorized representative of the United States in its relations with foreign states. The President accordingly has some inherent, though limited, lawmaking authority under Article II of the Constitution. The Article supports this doctrinal argument by examining the separation-of-powers concerns underlying the courts’ historic deference to executive branch determinations of foreign state immunity, prior to the codification of that subject in 1976. It considers objections that the Executive Branch’s authority to determine head of state immunity is more plausibly grounded in the Reception Clause than in the President’s more general power to conduct the nation’s diplomacy, and that head of state immunity determinations are not really lawmaking. The Article concludes by considering the respective roles of Congress and the courts in determining and applying principles of head of state immunity.