The majority of microplastics (MP) found in the marine environment have land-based sources and rivers are known as carriers of these particles to the ocean. This work is the first to study MP in the Lis River Basin and coastal shoreline (total nine sampling sites) and the potential relationship with inputs from 105 companies/units. We report the seasonal abundance, typology, colour, size, and polymer distribution of MP in surface water and sediment. To the collected samples, a digestion protocol was applied, followed by filtration. For sediment, an MP density separation was performed prior to filtration. MP were mostly fibres (0.02 to 1111.11 items.m⁻³ in water and 10.66 to 1609.64 items.kg⁻¹ in sediment) and fragments (0.02 to 2311.11 items.m⁻³ in water and 10.66 to 501.01 items.kg⁻¹ in sediment). The most frequent colours were transparent and blue, and MP were mostly smaller than 1 mm in water and 2 mm in sediment samples. The most abundant polymers in water were polyethylene (37%), polyacrylate (18%) and polystyrene (18%), and in sediment, polyethylene terephthalate (29%) and polyacrylate (23%). Population density, plastic processing companies and meteorological factors were found to be associated with seasonal MP abundance and distribution patterns in the Lis River Basin.