Climate change and rapid land-use change intensify floods and droughts in the Lower Mekong Basin. Vulnerability is particularly high in transboundary areas, where upstream-downstream interdependencies limit purely national adaptation. In the 9C-9T sub-basin shared by Cambodia (9C) and Thailand (9T) nearly 1.8 million people are affected. Facilitated by Mekong River Commission (MRC) and supported by GIZ, both countries established a Joint Governance Mechanism for Transboundary Assessment, Planning and Action. With a shared vision, the endorsed 9C-9T Flood and Drought Master Plan guides investments towards Nature-based Solutions (NbS). Economic analysis strengthened the case and enabled German funding for two pilots: (1) a constructed wetland in Thailand reducing wastewater flowing to Cambodia, and (2) rural retention ponds in Cambodia for water security and reduced flooding. Local Climate Risk Assessments (CRA) translate basin-scale concepts into site-specific Ecosystem-based Adaptation (EbA) measures now being implemented.