Natural source nanostructures have attracted a lot of attention due to their diverse biological activities and low toxicity to people, animals, and the environment. In this research, Penicillium chrysogenum AUMC 14831 isolated from Pancratium maritimum was used to reduce sodium selenite (Na₂SeO₃) to selenium nanoparticles (Se⁰), by reacting with fungal filtrate. Selenium nanoparticle characterization was performed using UV-visible spectroscopy and the color of fungal filtrate turned into ruby red when treated with sodium selenite and gave a peak at 300 nm based on its optical property and surface plasmon resonance. Nanoparticles' crystallinity was determined by X-ray diffraction (XRD) and indicated that the peaks were sharp and located at 2θ values 23.5°, 29.7°, 41.32°, 43.65°, 45.36°, 51.7°, and 65.22°. Transmission electron microscopy (TEM) demonstrated the form and SeNPsʼ size which appeared spherical with diameters between 9 and 18 nm. Fourier Transform Infrared Spectroscopy (FTIR) revealed the capping agent's functional groups in charge of the stability and activity of SeNPs. Biosynthesized selenium nanoparticles showed significant antibacterial and anticandidal action. Mycologically synthesized SeNPs are promising compounds as antimicrobial agents.