On June 23, 2017, the Supreme Court’s 5–3 decision in Murr v. Wisconsin provided some long‐awaited guidance for courts to use in defining the relevant unit of property to perform regulatory takings analyses. The Court announced a new three‐part test, tasking lower courts with two different multifactor inquiries to determine whether a federal regulatory taking has occurred. First, the court must figure out the right unit of property, and second, it must determine whether just compensation is owed using the three‐pronged test articulated forty years ago in Penn Central Transportation Co. v. City of New York. Despite its resort to multiple factors to determine the relevant unit of property, Murr is most striking because of an important one it minimizes: state‐specific positive law. Takings doctrine has long valued, incorporated, and awarded protection to varied unique forms of state property. This constitutional property federalism is widely perceived as desirable, encouraging beneficial competition and innovation in the forms and content of property rights in different jurisdictions. This Essay argues that the Murr decision threatens constitutional property federalism by inviting courts to define the scope of protected property interests using law and regulation across jurisdictional boundaries. As lower courts and scholars struggle to give meaning to Murr, its effects on the federalist structure of property should not be neglected.