The biosynthesis selenium nanoparticles (SeNPs) were prepared from aqueous extracts of Euphorbia retusa, and the active components of the plant extract were identified. Additionally, an examination was conducted to determine the antioxidant and antimicrobial properties of metal nanoparticles derived from this plant. The SeNPs were characterized using a variety of techniques, such as UV-visible, transmission electron microscopy (TEM), and zeta potential spectroscopy. As a result, the levels of phenolics, flavonoids, and tannins that were present in the nanoparticle samples that were created were lower than the levels that were present in the aqueous extract to begin with. The antioxidant activity of metal nanoparticles (IC50 = 0.247mg/ml) and E. retusa extract (IC50 = 0.054 mg/ml) was assessed making use of the DPPH assay. The findings for the plant extract showed much higher efficacy than those for the metal nanoparticle solutions, which showed significantly lower effectiveness. To determine whether the samples have antibacterial properties, they were tested against several bacterial and fungus species. The activity of the selenium nanoparticles against Escherichia coli, Klebsiella pneumoniae, Bacillus cereus, Bacillus subtilis, Staphylococcus epidermidis, and Candida albicans was shown to be the most powerful. In conclusion, the creation of selenium nanoparticles is an essential step in the process of enhancing the biological properties of an organism.