This study aims to better understand how plastic waste travels from land-based sources to marine environments by analysing the material flow of plastic waste in five high-priority catchments and three tourist hotspots in Thailand. This study also presents the first large-scale assessment in Thailand to integrate national waste generation and waste management performance data with actual hydrological conditions to estimate how mismanaged plastic waste is carried and discharged into the marine environment. By mapping the relationship between waste sources, leakage pathways and plastic discharges to the marine environment, this study identifies the most significant hotspots contributing to marine plastic debris and the specific associated waste handling practices.