A popular conservation tool in the wider Caribbean, marine protected areas are increasingly being managed through co-management arrangements, where government authorities share planning, management, and decision making responsibilities with local users and other stakeholders. Co-management arrangements are expected to improve the planning and management process and enhance social and ecological impacts of MPAs, yet few empirical studies have examined the relationship between co-management and MPA success. Using social and ecological data from a rapid assessment of twenty-eight MPAs and their associated communities in the wider Caribbean, this study investigates relationships between formalized co-management arrange-ments and measures of MPA success, which consist of stakeholders perceptions and measured ecological impacts. Findings indicate that formal co-management arrangements were not associated with stakeholders perceptions of the management process and social impacts, nor with measured impacts on fish and coral conditions; however, co-management arrangements were related to stakeholders perceptions of some ecological factors. These empirical results support claims in the resource management literature that simply formalizing a co-management arrangement does not necessarily lead to management success. A variety of factors associated with co-management can affect social and ecological performance of MPAs, and it is important that these factors be carefully considered in the design of co-management arrangements.