Coastal regions host a large portion of the global population and face complex balancing challenges for achieving economicEconomic, socialSocial, and environmental sustainabilityEnvironmental sustainability. This chapter presents an innovative participatory approachParticipatory approach to meeting these challenges, developed in the EU Horizon 2020 project “The collaborative land-sea integration platform” (COASTAL) and applied and tested in two Mediterranean coastal case studies of Mar Menor in SpainMar Menor Spainand South-Western MessiniaSouth-Western Messinia Greece in Greece. The chapter describes the COASTAL participatory approachParticipatory approach and the outcomes of its application to these two Mediterranean cases. This approach engages stakeholders from various sectors to identify key challenges and robust solution roadmaps for meeting them to achieve a sustainable coastal development. It further employs system dynamics modeling, scenario analysis, and solution robustness analysis to assess the effects of the stakeholder-prioritized solution roadmaps under uncertain future climate and socioeconomic conditions. The outcomes for the Mediterranean case studies are consistent in identifying rural-coastal management integration and synergy (such as promoting rural and coastal ecotourism and transitioning from conventional to more sustainable farming practices) as key solution components of a robust sustainable development roadmap. In both cases, a timely (even partial) implementation of the stakeholder-prioritized solution roadmaps is also found to be critical for improving local population well-being, meeting water availability and quality demands, protecting good ecological status in the coastal lagoons, and strengthening the regional resilience to uncertain external factors, such as forthcoming climate change and new future socioeconomic challenges.