AbstractIntegrating stakeholder knowledge, views and needs in marine or maritime spatial planning (MSP) processes is important from a governance and social sustainability perspective both for MSP practitioners and for the evolving field of MSP research. Transboundary MSP appears particularly challenging for participation, which is why it is important to identify opportunities and address obstacles for stakeholder integration in this specific context. This article examines how stakeholder integration is currently practiced in the Baltic Sea Region (BSR), an enclosed sea where policy coherence and addressing conflicting interests across borders are especially relevant. It synthesises a range of challenges and enablers for stakeholder participation and mobilisation that have emerged from two transboundary MSP research and development projects, BaltSpace and Baltic SCOPE. The article finds that with the exception of statutory authorities, stakeholder engagement in the BSR is mostly limited to self-motivated stakeholders and consultation rather than more inclusive forms of participation. This can reduce the quality and legitimacy of MSP processes and risks to concentrate power in the hands of a small group of actors. For transboundary stakeholder integration to become more interactive and effective, five types of challenges need attention, regarding a) timing, b) governance systems, c) capacity and processes, d) stakeholder characteristics and e) knowledge and language. These obstacles can be addressed by (1) a dedicated research and development agenda that critically reflects on integrative tools and processes, and (2) by encouraging transnational institutions in the BSR to devote more resources to transboundary stakeholder integration and adopt flexible and adaptive strategies and tools that can facilitate stakeholder involvement throughout the MSP policy cycle.