Wetland degradation, driven by pollution and encroachment, presents a significant global challenge. This study explores the multi-country efficacy of Floating Treatment Wetland Systems (FTWS) as a Nature-based Solution (NbS) for remediating urban water bodies across Bangladesh, India and Nepal, using a common experimental framework that integrates an initially controlled microcosm study, followed by field trials engaging the community members and researchers for their capacity building. Since FTWS uses low-cost raft construction, and locally available and recycled materials, this enables direct cross-country comparison and scalability of the FTWS. Experiments show reductions of up to 99% in nitrate (highest in Nepal), up to 91% in orthophosphate (highest in Nepal), and up to 96% in biochemical oxygen demand (BOD) (highest in India). Across all sites, Canna indica emerged as a highly effective species for FTWS due to its robust nutrient uptake and its ability to survive in diverse environments and conditions. Field trials conducted at Dhanmondi Lake (Bangladesh), Nayagaon Talai (India) and Nagdaha (Nepal) demonstrated significant improvements in water quality, with increases in dissolved oxygen of up to 50% and reductions in nitrate and phosphate concentrations. The study also emphasised capacity building efforts, engaging communities through workshops and training sessions, thereby enhancing awareness and support for FTWS technology. This multi-country initiative underscores the potential of FTWS as a cost-effective NbS for improving urban water quality while fostering community engagement and wetland conservation.