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This investigation aimed to characterize the green synthesized selenium nanoparticles using Sonchus maritimus L. extract and evaluate their antioxidant and antibacterial properties. Moreover, acute toxicity of nanoparticles was performed in Wistar rats. The synthesis of SeNPs was confirmed by Ultraviolet-visible, Fourier Transform Infrared Spectroscopy, Scanning Electron Microscopy and Energy Dispersive X-ray analysis. Antioxidant activities of S. maritimus and SmE-SeNPs were determined by DPPH and FRAP assays. Antibacterial activities were tested against Gram positive and negative pathogen bacteria. The SEM results showed that SeNPs had a spherule-like structure reaching up to 26.48 nm. In addition, S. maritimus and SmE-SeNPs had DPPH scavenging activity and reducing power. SeNPs exhibited activities against Escherichia coli and Staphylococcus aureus. The intraperitoneal toxicity test of SeNPs showed no mortality and minor behavioral variations. In conclusion, S. maritimus can be considered as biocatalyst stabilizers for the biosynthesis of SeNPs which might be used in several applications due to their biological efficiency.