Microplastics have been a global concern due to their potential and widespread risks to organisms and environments. In this study, we investigated the abundance, distribution, and characteristics of microplastics (MPs) in the surface waters of the Bahía Blanca Estuary (BBE), specifically in its inner and middle zone. The results showed the dominant shape of MPs were fibers, being black, transparent, and blue the main colors. The concentrations of MPs ranged from 182 to 33,373 items m–3 with a mean value of 6162 items m–3. The highest concentrations of MPs were detected in the middle zone of the estuary, a site that receives untreated sewage effluents from the city. The most abundant size ranges were from 0.5 to 1.5 mm (44.21%) and ˂0.5 mm (40.21%) and were predominant at all the sampling sites. The concentration of mesoplastics in the inner zone (16 items m–3) presented larger values than in the middle zone (5 items m–3). A wide variety of polymeric materials with predominance of microfibers such as cellulose-based, polyacrylonitrile, polyethylene terephthalate, and polypropylene were identified. Polyester/alkyd resins and poli(vinyl chloride) were also found. The analysis of MPs surface through SEM/EDX detected a variety of elements such as C, O, Si, Al, K, Ca, Cl, Ti, Fe, S, and P, indicating potential contaminant carriers in the water column. Some plastic particles presented a high degree of degradation on their surface morphology. Untreated sewage discharges appear to be a significant input of MPs. Therefore, the results provided in the present study should be considered by stakeholders interested in the management and conservation of this large coastal wetland with significant ecological and economic value. Display omitted •A mean of 6162 items m-3 was found in the surface waters of the estuary.•Cellulose, polyacrylonitrile, and polyesters were the most common polymers.•Mesoplastics (˃5 mm) were more abundant in the inner part of the estuary.•Black and transparent microplastics were the most conspicuous.•Sewage waters seemed to be the main input source of microplastics pollution.