Current studies on emerging pollutants have demonstrated the abundance of Microplastic (MPs) in coastal environments around the world and the austral Buenos Aires Province coasts are not the exception. Sampling locations included two National Harbours in the nearness (Villa del Mar and Arroyo Parejas), oil transport facilities, industrial and recreational/tourism activities (Pehuen-co), artisanal fishing and not completely treated wastewater discharges which set a proper scenario for the invasion of the alien species Pacific Oyster (Crassostrea gigas). Then, for the first time, we analysed the occurrence of MPs in oysters from 23 composed samples collected between November 2021- April 2022 which were kept at -20 °C until hydrogen peroxide treatment (30% H2O2) and filtration (GF/F, 47 mm and 0.7 µm pore). Microplastic items were observed under a Nikon SM-Z1500 lens. In total, 237 items were counted in 115 individuals, with an average size of 1360 μm. Total MPs levels ranged from 0.00 items/g to 2.82 items/g (wet weight) (average 0.22 ± 0.57 items g–1) particles/g wet weight in the oysters. The dominant shape of MPs were fibers (98.3%) while seldomly, films structures were found in the oysters. The predominant color was yellow and black (25.35% and 22.75%, respectively), followed by white and blue (17.17% and 16.31%, respectively). Arroyo Parejas Beach and Puerto Belgrano were the coastal locations wich showed the highest rate of microplastic ingestion by oysters. This research provides the first baseline data on microplastic contamination status in oysters in a relevant environmental area from the Western South Atlantic coastline. Considering the marked advance of the Pacific oyster on these coasts and its negative impact in itself (loss of beaches, injuries to bathers and intoxication due to consumption), it is relevant to monitor the content of microplastics given the implications to food security and potential risks to human.