Rivers act as pathways for plastic litter but also constitute major accumulation zones. In the Seine estuary, several accumulation zones where macroplastics can have residence time up to decades are well known, which raise the question about the production of microplastics within these sites. To answer this question, 9 sites with different dynamics of accumulation and remobilization were studied by sampling mud (n = 13) in their intertidal zones and soil or sand (n = 15) in their supratidal zones. Samples were taken either in the macroplastics patches or around. Microplastic (> 25 µm) were analyzed by µFTIR imaging in transmission mode and with a 25 µm x 25 µm pixel resolution. Results demonstrate high contamination levels with concentrations ranging from 300 to 230 000 MP/kg of dry matter. Between 1 and 10 different polymers were identified according to samples. Polystyrene is the most concentrated polymer. Except for one site, microplastic concentrations are higher in soil samples than in mud samples (median values equal to 30 000 and 5 000 MP/kg, respectively). This tendency may result from a greater accumulation and a longer residence time of microplastic the supratidal zones than in the intertidal ones, but also from the degradation of macroplastics at the surface of these soils. In fact, whatever the sample types, microplastic tend to be more concentrated and coarser in the macroplastic patches than around. This trend is even more visible in the sites where macroplastic remobilization is the lowest.