Rivers are important routes for sea-bound microplastics. Thus, this study investigated the occurrences and distributions of microplastics and polychlorinated biphenyls (PCBs) in sediment samples from the Qiantang River (QR) and Hangzhou Bay (HZ) and analyzed the correlation between microplastics and PCBs. A total of 15 sampling sites were selected, including eight from the QR (i.e., four in the Tonglu area and four in the Fuyang area), two from the Andong salt marsh (ASM; located in a hydrodynamic turning point of the HZ), and five from HZ. The mean microplastic abundance was highest in the QR, followed by HZ and ASM, with 0.23 ± 0.06, 0.18 ± 0.05, and 0.15 ± 0.03 particles/g sediment, respectively. Cluster analysis demonstrated that fragments and fibers may have originated from domestic sewage inputs to the QR. Spatially, mean PCB concentrations from Tonglu, Fuyang, and HZ were 1.47 ± 0.10, 1.65 ± 0.10, and 1.65 ± 0.09 ng/g sediment, respectively, which were higher than that from the ASM (1.13 ± 0.09 ng/g sediment). The vertical distributions (0–5 cm, 5–10 cm, and 10–15 cm) of microplastics in the sediments at Tonglu and Fuyang decreased with increasing depth, which was opposite to the depth trend of PCB concentrations. Micro-Fourier transform infrared spectroscopy analysis suggested that polyethylene was typically the dominant polymer, accounting for 60 ± 0.08% of the total suspected plastic particles. Microbeads and films showed considerable correlations with both highly and lesser chlorinated PCBs. Overall, our findings highlight the need for routine monitoring of microplastics in China in addition to measures for controlling plastic pollution on a national scale. Further study should ascertain specific sources of microplastics and analyze their adsorption capacity to organic pollutants.