Selenium nanoparticles (SeNPs) are well‐known bioactive compounds. Various chemical and biological methods have been applied to SeNP synthesis. Spirulina platensis is a widely used blue‐green microalgae in various industries. In this study, the biosynthesis of SeNPs using sodium selenite and Spirulina platens has been developed. The SeNP synthesis was performed at different cultivation condition including pH and illumination schedule variation. The SeNPs were characterized by FT‐IR, XRD, size, and zeta potential measurements, and the antioxidant activities of selected SeNPs were evaluated by DPPH and FRAP assays. FT‐IR analysis showed the production of SeNPs. The 12 h dark/12 h light cycles and continuous light exposure at pH 5 led to the production of stable SeNPs with sizes of 145 ± 6 and 171 ± 13 nm, respectively. Antioxidant activity of selected SeNPs was higher than sodium selenite. It seems that green synthesis is a safe method to produce SeNPs as well as a convenient method to scale‐up this production.