Plastic debris and microplastics (MPs) pose an escalating environmental threat, largely driven by urbanization and industrialization. Over 80% of these pollutants originate from land-based sources and ultimately find their way into our oceans. Southeast Asia (SEA) is recognized as a critical hotspot for plastic waste discharge into marine environments. However, significant knowledge gaps persist, particularly regarding the source-sink relationships and the mechanisms through which MPs enter marine ecosystems, especially from highly urbanized areas. Current studies often fall short by focusing on specific environmental compartments or linear pathways, neglecting the complex interdependencies among various environmental compartments and pathways in aquatic ecosystems. Consequently, a comprehensive systems-based model is essential to address these gaps, encompassing major land-based sources, aquatic pathways, and environmental sinks. This project aims to develop a holistic framework to quantitatively assess MP influx in urban aquatic systems, addressing critical knowledge gaps in MP management within selected urban areas of SEA. Key activities will include critical literature reviews, field data collection, quantitative influx analysis, and engagement with policymakers. The anticipated outputs of this project include evidence-based policy recommendations for relevant stakeholders, including local governments and environmental management agencies, to formulate effective strategies for preventing marine plastic pollution.