The extensive use and mismanagement of plastic causes environmental pollution in air, soil, and water and threatens ecosystem health and functioning. A large part of plastic in the environment is microplastic (MP), small particles ≤5 mm. To advance the understanding of MP pollution in rivers and anthropogenic environments and to provide new insights into the fate of small (25-300 μm) and large (300-5’000 μm) MPs, we investigate MPs in Greater Paris and the Seine river catchment in France. To compare MP concentrations upstream and downstream of Paris we carried out three out of four samplings at seven sites along the Seine river and tributaries in different flow seasons using a novel in-situ pump and cascade filtration system to increase representativity by sampling larger water volumes up to one m3. Preliminary data analysis of samples collected upstream and downstream of Paris indicate increased MP concentrations and higher polymer type variation downstream compared to upstream. Among others we detected PP (polypropylene), PE (polyethylene), PS (polystyrene), PVC (polyvinyl chloride), Polyester (including PET), SBR (styrene butadiene rubber), PA (polyamide), PU (polyurethane), Acrylic and ABS (acrylonitrile butadiene styrene). The findings confirm the hypotheses that urban agglomerations may contribute to MP concentration in surface waters. This trend needs to be confirmed by final data analysis including all samples and size fractions and in combination with biogeochemical parameters. Final results are foreseen to be completed in 2022. These studies can help to identify sources and pathways of microplastics in rivers in urban agglomerations and help policy and decision makers to address environmental pollution and mitigation actions.