The Article reviews the constitutional status of sister-state sovereign immunity. It argues that the parity requirement announced in Franchise Tax Board v. Hyatt (2016) is a temporary compromise that is supported by neither the purposes of the Full Faith and Credit Clause nor by cases cited by the Court. It further argues that parity is bad policy because parity overprotects states for acts they commit beyond their borders and under protects the interests of forum states in regulating conduct within their territorial jurisdiction. But the Article breaks from most scholarship. It suggests that the Court went too far in Nevada v. Hall (1979) in finding that nothing in the Constitution compels states to respect sister-state claims to sovereign immunity. But it does not endorse those critics who find absolute state immunity in policies of federalism. Instead it proposes a limited constitutional basis for sister-state immunity that grounds this immunity in territorial restrictions on judicial power that operated during the founding era. Under the proposed approach, states would enjoy sovereign immunity in a sister-state court—but only for acts they or their agents commit in their own territory. The Article explains how this limited immunity accommodates the competing interests of the states, and why it is superior to alternative proposals to ground sister-state immunity in international law