Due to the discharge of industrial wastewater, urban domestic sewage, and intensive marine aquaculture tailwater, nitrate (NO3-) pollution has emerged as a significant issue in offshore waters. Nitrate pollution affects aquatic life and may interact with other pollutants, leading to comprehensive toxicity. Cadmium (Cd2+) is the most widespread metal contaminant, adversely affecting aquatic life in the coastal waters of China. Despite this, few studies have focused on the synergistic toxicity of NO3- and Cd2+ in marine organisms. This study conducted a 30-day exposure experiment on marine Japanese flounder (Paralichthys olivaceus) to explore the synergistic toxicity of NO3- and Cd2+. Our results demonstrated that the exposure to Cd2+ alone induced slight histopathological changes in the liver. However, malformations such as hepatic vacuolar degeneration and sinusoid dilatation were exacerbated under co-exposure. Moreover, co-exposure induced the downregulation of antioxidants and the upregulation of the product malonaldehyde (MDA) from lipid peroxidation, indicating potent oxidative stress in the liver. The increased mRNA expression of IL-8, TNF-α, and IL-1β, along with the decreased expression level of TGF-β, indicated a synergistic inflammatory response in the organisms. Furthermore, the co-exposure led to an abnormal expression of P53, caspase-3, caspase-9, Bcl-2, and Bax, and disturbed the apoptosis in the liver through TUNEL staining analysis. Overall, our results imply that co-exposure synergistically affects inflammation, redox status, and apoptosis in flounders. Therefore, the findings from this study provide valuable perspectives on the ecological risk assessment of marine teleosts co-exposure to NO3- and Cd2+.