This study investigates the use of Excoecaria agallocha leaves as a bio-template for the intercalation of Selenium nanoparticles (SeNPs). The synthesized SeNPs were characterized using techniques like SEM-EDX, TEM/HR-TEM, and XRD spectroscopic studies. The study found that SeNPs showed maximum cleaning ability at a dosage of 50 μl/mL, with 95% inhibition of DPPH radicals. However, cellular absorption was limited to 55% at concentrations of 300 μg/L over a 72-h period. The synthesized SeNPs also demonstrated a strong cytotoxic effect on MCF-7 breast cancer cell lines, indicating their potential as anti-cancer agents. Further research is needed to fully explore the potential of these novel nanocomposites.