Mussels can accumulate microplastics (MPs) present in seawater and are one of the species most affected by MP pollution. This study is the first to evaluate the abundance of MPs in the small mussel Brachidontes rodriguezii at four stations (S1, S2, S3, and S4) with different levels of human activities along the intertidal area of the most popular resort city of Argentina (Mar del Plata, Buenos Aires). Microplastics, primarily microfibers, were detected in 97.5% of the analyzed mussels by visual identification. The abundance of MPs varied significantly among the stations, with the highest levels observed in mussels from S4, corresponding to the low-urbanized area. This finding seems to suggest that factors other than urban pollution, such as agricultural activities and nearby streams, may contribute to MP contamination. The study also found a relation between MPs abundance and the mussels’ condition index, suggesting that high levels of MPs may negatively impact the health of these organisms. Identification suggested that all found microfibers were plastic, with approximately 10% of the analyzed microfibers revealing the presence of polymers such as polyester, polychloroprene, polyacrylonitrile, and polyethylene terephthalate. For several microfibers, only the pigments but not the substrate could be identified, and about half of the microfibers were Raman inactive, thus limiting definitive identification. These findings highlight the widespread MPs contamination in marine environments and the use of mussels as bioindicators of MP pollution. Future research should focus on identifying the sources of MPs, assessing their potential ecological impacts, and developing effective strategies for mitigating MP pollution.