Even though marine protected areas (MPAs) have become central instruments in the endeavour towards sustainable development, our knowledge on how different institutional designs influence outcomes is limited. Using a comparative case study design, this paper explores the interplay between institutional arrangements and management outcomes in two adjacent yet institutionally slightly differing MPAs, encompassing a shared marine trench and a partially inhabited archipelago landscape – namely the Koster Sea National Park in Sweden and the Outer Hvaler National Park in Norway. How can differences in the institutional designs governing the two parks, be linked to differences in sustainability outcomes? What lessons can be learnt for the design of MPAs? The study shows that institutional design influences management outcomes in some respects but not in others. Differences in overall management systems had no noticeable effects on sustainability outcomes and how they were perceived, while the differing objectives of the parks and how they are made operational seem to have affected the outcomes. But they have also influenced actors' expectations and their assessment of outcomes. According to this study, conservation arrangements can be broadened beyond mere nature protection. However, the study also underlines the challenges of locally adapted and participatory institutional designs and emphasises the importance of taking users’ varying expectations related to social and economic values into account throughout the whole process. The establishment of national parks is no guarantee for broader sustainable development per se; this also requires resources and proper embedding and integration with relevant sectors and tiers in the overall management system.