BACKGROUND: Nowadays, researchers are attracted to the phyco-synthesis of selenium nanoparticles (SeNPs) for biotechnological and medical applications as they possess many advantages such as safety, nutritional value, and easy biodegradation than gold, copper, and silver nanoparticles. Spirulina platensis is the preferred microalgae for SeNPs synthesis because it contains many compounds that increase their stability making them fit for biomedical treatments. RESULTS: The biosynthesized Spirulina platensis selenium nanoparticles (SP-SeNPs) were spherical and crystalline, with a diameter of 65 nm and a net charge of -16.7 mV. Furthermore, they were surrounded by active groups responsible for stability. The DPPH radical scavenging test assessed the antioxidant efficacy of SP-SeNPs and exposed scavenging inhibition of 79.234% at a 100 µM dosage. ABTS and H CONCLUSION: The investigation findings reveal that SP-SeNPs are a hopeful antimicrobial, anti-tumor, anticoagulant, antioxidant, and anti-inflammatory factor that can be applied in medical cures.