Zinc oxide nanoparticles (ZnO-NPs) are widely used in industrial and agricultural applications in addition to cosmetic products. However, the extensive use of ZnO-NPs can impose serious environmental problems on the aquatic ecosystem. So far, the hazardous impact of ZnO-NPs on phytoplankton is not well studied. Among myriad aquatic microorganisms, microalgae are sensitive indicators for water pollution. The current study investigated the toxic effects of different concentrations of ZnO-NPs (10-200 mg/L) on growth, metabolic profile, and morphology of the green microalga, S. opoliensis. The results revealed that ZnO-NPs significantly decreased cell growth, chlorophyll-a (Chl-a), and carbohydrate content with a concentration-dependent manner. In contrast, the protein and lipid contents were progressively increased after ZnO-NPs treatment compared to the control condition. The morphological examinations revealed obvious changes in the microalgal cell particularly at high ZnO-NPs concentrations. The present study provides a new report on the ecotoxicology of ZnO-NPs contamination on the aquatic ecosystem and highlights its toxic impact on S. opoliensis