Mediterranean islands face significant environmental challenges due to their high population density, reliance on imports, and water scarcity, exacerbated by increasing risks from climate change. Nature-based solutions (NbS) could address these challenges sustainably and with multiple benefits, but their uptake in policy and planning is limited, and stakeholder perspectives are conspicuously lacking from current research. Here, we report the results of a collaborative, multi-stakeholder exercise to identify priority knowledge needs (KNs) that could enhance the uptake of NbS in Mediterranean islands. We used a well-established iterative prioritisation method based on a modified Delphi process. This was conducted by the authors, environmental policy and practice stakeholders from across the Mediterranean islands, representing business, government, NGOs and research. We developed a long list of potential KNs through individual submissions, and prioritised them through voting, discussion and scoring. Excepting workshop discussion, all individual contributions were anonymous. We present the 47 resulting KNs in rank order, classified by whether they can be addressed by knowledge synthesis and further research, or demand action in policy and practice. The top priority KNs are i) a more precise definition of NbS, ii) which NbS are adapted to dry Mediterranean conditions? iii) how to increase the adoption and use of NbS in urban plans?, iv) how can buildings and built-up areas be modified to accommodate green infrastructure and v) cost-benefit analysis of urban green spaces. In collaboration with these stakeholders, our findings will determine future research strategies on NbS implementation in the Mediterranean islands.