We assessed microplastic (63-1000 µm) pollution in sediments and superficial and bottom water samples during the dry and rainy seasons (Oct/2018 and Mar/2019, respectively) from the Guarapiranga Reservoir (Metropolitan Region of São Paulo, Brazil), which supplies drinking water for 3.8 million people. The concentration of microplastics varied spatially and seasonally, where the higher concentrations observed near the heavily urbanized areas and during the dry season. Water column concentrations ranged from 150-3100 particles/m3 water and 0.07-25.05 mm3 plastic/m3 water (dry season), 70-7900 particles/m3 water and 0.06-4.57 mm3 plastic/m3 water (rainy season). Sediment samples were collected only during the rainy season, and concentration ranged from 210-22999 particles/kg dw and 0.15-111.46 mm3/kg dw. Microplastics concentration was generally lower in the rainy season, but it seems that the higher reservoir level exerted a considerable influence by changing the residence time and compensating by “dilution” the higher input of microplastics during the rainy season. Therefore, we suggest investigating the relationship between microplastic flux, microplastic concentration, and residence time to advance knowledge on microplastic pollution dynamics in lentic environments. Moreover, our results may contribute to creating and improving local monitoring programs regarding microplastic pollution and adopting specific public policies.