Countries exploiting transboundary fisheries face strong incentives for over- exploitation. This basic economic insight has been validated empirically; transboundary fisheries tend to be in worse condition than fisheries in single nations. Thus, transboundary fisheries pose a significant, and globally ubiquitous, management challenge. Attempts to solve this challenge through cross-country cooperation have been largely unsuccessful because defection is often more attractive than adhering to cooperative agreements. We explore the economics of an alternative solution, a transboundary marine protected area (TMPA), and derive the conditions under which it can improve profits and stock biomass, even in the presence of individually-rational non- cooperation across countries. We find that well-designed TMPAs have the potential to overcome non-cooperation across countries; this result is strengthened when stocks have relatively low growth rates. A well-designed TMPA can earn higher profit for both countries, increase stocks in both countries, and reproduce the fully cooperative outcome.