This article analyses how Biosphere Reserves (BRs) in the Biosphere for Baltic network operate as Boundary Organisations (BOs) in support of collaborative landscape governance. Using Ostrom's Institutional Analysis and Development framework in combination with BO theory, it examines how institutional arrangements, stakeholder interactions, and contextual conditions shape the capacity of BRs to convene, translate, collaborate, and mediate across governance levels. Based on case studies of seven BRs in the Baltic Sea region, the results show consistent patterns in how governance models, participatory processes, knowledge flows, and institutional recognition enable or constrain boundary functions. Effective convening and collaboration were most evident where long-standing relationships, strong trust, and cross-sectoral platforms were in place. Barriers arose from short-term project cycles, uneven participation, tensions around authority and role boundaries, and limited national-level uptake of BR-generated knowledge. Knowledge translation appeared stronger when BRs could connect scientific, local, and administrative perspectives and relate them to planning and management needs, while institutional misalignment made learning and follow-up more difficult. The findings underscore the need to balance institutional flexibility, neutrality, learning, and embeddedness to enhance the effectiveness of BRs as boundary organisations for advancing landscape governance.