Economist Paul Romer proposes the establishment of charter cities. Charter cities would resemble special economic zones; that is, small regions that experiment with economic rules that differ from those governing their larger ‘host’ countries. Yet unlike a special economic zone, a charter city would also experiment with its own legal and political rules. The rules, in turn, can be enforced by a third-party coalition of representatives of foreign countries that enforce these rules at home. Host countries that face problems of economic stagnation or political instability can thus leverage the experience and credibility of ‘guarantor’ countries to gradually reform their own institutions. I defend charter cities as an instrument of cosmopolitan justice in nonideal global conditions. Contributing to efforts to establish charter cities is, in many cases, a more efficient use of a country's scarce resources than foreign aid or attempts to facilitate institutional change abroad. I argue that countries also have a moral reason to contribute to charter cities: they can serve as compensation for the coercive harms due to those countries' border closures. I conclude that cosmopolitans and nationalists can find common ground in their reasons to support the establishment of charter cities.