When does a state have standing to challenge the Executive Branch’s alleged underenforcement of federal law? The issue took on importance during the Obama Administration, with “red states” suing the Executive Branch over numerous issues, including immigration and health care. The question of state standing has already appeared in important litigation during the first months of the Trump Administration, only with the political orientation of the actors reversed. This Article argues in favor of sovereign preemption state standing, under which a state would enjoy Article III standing to sue the federal government when (1) the federal government preempts state law in an area, yet (2) the Executive Branch allegedly underenforces the federal law that Congress enacted to address that very same area. Sovereign preemption state standing arises naturally out of the function of states in the federal system. It is grounded upon parens patriae injury—that is, injury to the state’s ability to protect its citizens against harm. The federal government can properly preempt state law, on the logic that it then assumes from the state the obligation to protect the state’s citizens from harm. Where the Executive Branch then fails adequately to enforce federal law, it leaves the state’s citizens unprotected. The state then has Article III standing to sue the federal government on behalf of its citizenry. The universe of cases where sovereign preemption state standing operates is not large, which should assuage concerns over opening the floodgates of state–federal litigation. Moreover, prudential doctrines can be overlaid such that more cases would be screened out. Although sovereign preemption state standing could conceivably extend to Executive Branch overenforcement, such an application would not square with the functional justification for the doctrine.