© 2021 Elsevier LtdLevels, composition and fate of microplastics (MPs) were investigated along different compartments of a secondary wastewater treatment plant (WWTP) with nutrient removal on the northern Sea of Marmara coast (Istanbul, Turkey). When all samples were combined, fibers were found to be the most dominant particles, followed by hard fragments. 500–1000 μm and 1000–2000 μm were the most common size ranges for wastewater and sludge, respectively. Rate of removal differed for sizes and shapes of the particles combined. Hard fragments of <500 μm and fibers of size ranges 250–500 μm and 1000–2000 μm were more successfully removed within the WWTP. Size averages increased throughout the WWTP units. 84.6–93.0% removal was achieved for grab and 3-hr composite samples. Despite the high removal rates of the WWTP, 2,934 × 106 microplastic particles/d were released in the effluent to the Sea of Marmara. Our results show that the Ambarlı WWTP considerably contributes to microplastics contamination in the Sea of Marmara since the plant has a high operating capacity.