The diversity and intensity of human activities in the North Sea region (NSR) and other maritime regions are increasing. This necessitates transboundary coordination at the sea basin level, which is required but yet insufficiently established. Through European co-funded projects, national policymakers, stakeholders, and scientists in MSP are enabled to develop transboundary coordination (TBC) mechanisms. TBC requires, alongside other factors, a form of social and policy learning between these actors in different countries. The NorthSEE project (2016–2022) was an example of such an EU-co-funded project and was aimed at enhancing coherence in MSP processes and plans across the NSR. This article examines the project's key learning outcomes, the role of the MSP Challenge Simulation Platform in supporting these outcomes, and factors that enabled or constrained learning within the project. Data was collected during the project via document analysis, questionnaires from participatory stakeholder workshops, interviews with sixteen project participants and ten workshop participants, and observations. The study highlights that project participants have gained more insight into each other's planning systems, are able to contact each other more easily, and have initiated several follow-up initiatives. Furthermore, it shows that interactive and participatory tools, such as the MSP Challenge Simulation Platform, can contribute to individual and social learning by providing participants with instant feedback on their decisions. These learning outcomes have been influenced by various enabling and constraining conditions, including time, resources, and the differing levels of expertise and knowledge among project partners and participants. Assessing the broader societal impact remains a challenge and warrants further attention.
