Plastic pollution has become a serious environmental problem on a global scale. Microplastics (MP), in particular, pose a significant threat to marine ecosystems. This study assesses the abundance, distribution, and characteristics of microplastics (MP) present in the Sea of Marmara, where the specimens of selected mollusk (Ostrea edulis, Mytilus galloprovincialis, Rapana venosa), fish (Mugil cephalus, Pomatomus saltatrix), and sediment were investigated. A total of 292 specimens were examined. Microplastics are ubiquitously distributed in all environmental matrices and averaged 2.6 ± 0.9 MP per individual in mollusks and 2.8 ± 1.0 MP per individual in fish. In sediment, microplastics ranged from 195 to 226 MP per kg dry weight. The most abundant microplastics are fibers, blue, and polyethylene terephthalate. The results indicate that microplastic pollution is widespread throughout the Sea of Marmara and may bioaccumulate the food web. Further research is necessary to determine the longer-term trends as well as the ecological impacts arising from this pollution. Monitoring studies of key bioindicators, including bivalves and fish, as well as sediment analysis, are required to understand and mitigate risks to this critical marine environment from microplastics.