Nanoplastics have become an emerging contaminant in water bodies that adversely affects aquatic biodiversity. The present study investigated the effect of different concentrations of polystyrene (PS) nano-plastics (NP), 500 nm in size, on green microalgae Chlorella pyrenoidosa in terms of its growth, chlorophyll-a synthesis, oxidative stress , and cell viability . The morphological and compositional alterations in microalgae were observed using scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) analysis. Seven concentrations (1, 10, 100, 200, 500, 1000, and 5000 mg L −1 ) of PS NP were used for experimentation. Overall, a negative trend in the growth of C. pyrenoidosa was observed with increasing nano-plastics concentration. The chlorophyll-a synthesis was reduced to 3.0 %, 15.6 %, 25.9 %, 30.1 %, 37.6 %, 39.2 %, and 52.8 %, respectively, in all seven concentrations compared to control. The results of the cell viability assay confirmed that PS NP was toxic to microalgal cells and enhanced the production of reactive oxygen species. The results suggested that higher concentrations (1000 and 5000 mg L −1 ) were more toxic to microalgal cells. Furthermore, FTIR analysis revealed substantial alterations like nucleic acid degeneration at 1262 cm −1 in biomass exposed to 1000 and 5000 mg L −1 PS NP compared to other concentrations. Enhanced extracellular polymeric substances (EPS) secretion of 157 mg g −1 and 253 mg g −1 by 1000 and 5000 mg L −1 PS NP exposed cells, respectively, compared to control i.e., 92.2 mg g −1 was also noticed. SEM images showed aggregated nano-plastics adsorbed on the microalgal surface, whereas Transmission electron microscopy (TEM) micrographs revealed the internalization of nano-plastics with a slight deformation in the cell wall at higher concentrations (1000 and 5000 mg L −1 PS NP). Conclusively, higher concentration leads to high exposure risk, negatively impacting cellular functionality and their metabolic secretions. • Exposure to 1000 & 5000 mg L −1 polystyrene nano plastics reduced the cell viability by 24.3 & 38.9 %. • FTIR showed nucleic acid degeneration in biomass at nano plastics concentrations of 1000 & 5000 mg L −1 . • SEM confirmed adsorption and aggregation on the cell surface. • TEM revealed the internalization behavior of polystyrene nano plastics.