Selenium is a crucial trace element and the building block of many proteins. Humans who are deficient in selenium are more susceptible to myodegenerative disease, cardiovascular disease, and other illnesses. A small amount of selenium is required by the body to treat several disorders. Nano-selenium's easy cell penetration gives it a more powerful effect. When compared to chemically produced nanoparticles, biologically produced nanoparticles displayed lower toxicity. Mulberries are a rich source of vitamin C and iron, and previous studies have shown that they have a variety of pharmacological properties that include effects that are anti-inflammatory, anti-obesity, anti-hypertensive, anti-oxidative, anti-cancer, anti-atherosclerotic, and cardioprotective. In this study, nanoparticles were synthesized from mulberry leaves and characterization of selenium nanoparticles was done using analytical methods such ultraviolet-visible spectroscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction. These selenium nanoparticles were assessed for biological activities such as antioxidant, antimicrobial, and antitumor effects. Results demonstrated that selenium nanoparticle synthesized from mulberry showed particle size of 397nm in Scanning electron microscope analysis. In X- ray diffraction, nanoparticles showed broad peak at 2? angle of 20-250 which means it is crystalline in nature.  Fourier transform infrared spectroscopy analysis confirmed presence of alcohols, carbonyl group, methyl group, aldehyde group, esters, amine, ketones, amines were responsible for the formation selenium nanoparticles from mulberry leaves. The selenium nanoparticles exhibited maximum antioxidant activity in 2,2-Diphenyl-1-picryl-hydrazyl-hydrateassay were 48.57µg/ml when compared to control was 46µg/ml. In 2,2?-azino-bis3-ethylbenzothiazoline-6-sulfonic acid assays, IC50 of control was 49.3µg/ml and sample were 48.08µg/ml. SeNP showed IC50 of 49.08µg/ml and control were 43.5µg/ml in Nitric Oxide assay. In ferric ion reducing antioxidant power, IC50 value of nanoparticles was 41.069µg/ml and control were 41.54µg/ml. Maximum antifungal activity against Aspergillus niger at a concentration of 400µg and antibacterial activity against Staphylococcus aureus and Streptococcus mutans was at 500 µg of concentration was observed. Utmost antitumor activity was observed in 25µl of nanoparticles with an inhibition percentage of 87.96% against MCF-7 (breast cancer cell lines). The main objective of this study is to synthesize selenium nanoparticles by biological method. These nanoparticles can be effectively used in the drug delivery system to increase the bioavailability and efficacy of the drug molecule against various diseases.