Nanoparticles (NPs) are highly reactive particles that find a broad array of applications in society; as a result, they may accumulate in aquatic environments. Microalgae are the base of foodwebs and serve as surfaces for NPs, interacting and transporting them to higher trophic levels. We aimed at understanding the effects of environmentally significant and higher copper nanoparticles (Cu-NPs) concentrations on the photosynthetic performance of Chlorella sorokiniana, which included determining effective quantum yield (F’v/F’m), photochemical (Qp) and non-photochemical (NPQ) quenching, rapid light curves (RLC), light use efficiency (α), minimum saturation irradiance (Ek) and the maximum relative electron transport rate (ETRrmax), in addition to chlorophyll a content and cell viability. Dissolved Cu and free ionic Cu concentrations were determined, and dissolved Cu values were used as surrogate for nominal Cu-NPs concentrations. The experiments lasted 72 h and were carried out under controlled conditions. The results showed that cell viability and chlorophyll a decreased as Cu-NPs increased, but the opposite was obtained for the RLC parameters. Light saturation (Ek) and ETR increased as Cu-NPs concentrations increased in culture medium, but not the efficiency with which C. sorokiniana used the light. By showing that environmentally relevant Cu-NPs influenced C. sorokiniana metabolism, we add to the knowledge on the interactions between environmentally realistic Cu-NPs levels and phytoplankton cells.