© 2022 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of SciencesSince nanoparticle synthesis via chemical and physical methods is expensive and includes hazardous chemicals, biosynthesis has emerged as an environmentally friendly, clean and viable alternative. The present study reports the synthesis of selenium nanoparticles (SeNPs) using Citricoccus sp. For the production of SeNPs, the influence of some parameters (time, pH, temperature and stirring rate) was studied. Optimum synthesis conditions were found as pH 8, 24 h reaction time, 37 °C and 150 rpm. Synthesized particles were spherical and were 104.46 ± 50.82 nm with a zeta potential of –20.43 ± 0.41 mV. Afterward, the effects of the nanoparticles on oxidative stress biomarkers, hydrogen peroxide (H2O2), malondialdehyde (MDA), chlorophyll (a + b) and growth rate, subsequent to UV-C irradiation on Chlorella vulgaris were investigated. In culture contains nanoparticle and UV-C exposed, the amount of H2O2 and MDA decreased on the 1st, 3rd and 7th days following UV exposure compared to UV-applied group, while optical density and cell density increased, the amount of chlorophyll (a + b) changes were statistically similar. Consequently, it has been shown that the synthesized SeNPs alleviated the adverse effects of UV stress in microalgae.