**Shared Governance Improves Marine Protected Area Effectiveness****Overview**This research investigates the relationship between governance structures and the ecological outcomes of marine protected areas (MPAs), with a focus on comparing shared and state-based governance arrangements. Using global data, the study finds that MPAs with shared governance arrangements, where management authority is shared among multiple actors, exhibit improved fish biomass compared to those governed solely by state agencies.**Key Findings*** MPAs with shared governance had 98% higher fish biomass than MPAs governed by state agencies (Model 1).* Subnational, collaborative, and joint governance arrangements all had higher fish biomass than nationally managed MPAs under state governance (Model 2).* Greater MPA age and a higher proportion of no-take zones were also associated with increased fish biomass.* Notably, the positive effect of shared governance on fish biomass was stronger than the effects of MPA age and no-take regulations.* Other MPA and country-level covariates had minimal effects on fish biomass.**Implications for Management**The findings highlight the importance of shared and decentralized governance in marine conservation. Shared governance arrangements foster participation, collaboration, and local ownership, leading to more effective and equitable MPA management. They also increase management capacity, resilience, and legitimacy, which can result in greater ecological benefits.By incorporating diverse viewpoints, knowledge, and resources, shared governance can support pluralistic decision-making, adaptive management, and the articulation of user rights and interests. This inclusive approach can address potential negative impacts on local resource users and foster support for conservation initiatives. For example, in American Samoa, a Community-based Fisheries Management Program has successfully promoted compliance and collaboration by empowering local village residents as enforcement agents.**Shared Governance Considerations**It is crucial to acknowledge that shared governance is not a panacea, and its implementation requires careful consideration of the existing social and ecological context. Devolved governance arrangements should ensure appropriate representation of stakeholders, transparency, accountability, and fair decision-making processes. They should also be complemented with adequate investment in training, capacity building, and resource allocation to ensure the meaningful involvement and efficacy of local actors.**Conclusion**The study demonstrates that shared and decentralized governance arrangements play a significant role in achieving improved ecological outcomes in MPAs. By embracing inclusive and participatory governance models, protected area managers can foster collaboration, strengthen local ownership, and maximize the conservation impacts of protected areas. This approach is particularly relevant as global efforts intensify to expand the coverage of MPAs towards the ambitious goal of protecting 30% of the ocean by 2030.