Nanotechnology is significantly revolutionizing world agriculture through engineered nanomaterials, which contribute to enhancing agricultural production by controlling fungal and bacterial phytopathogens and, consequently, minimizing crop losses. In this study green silver nanoparticles (AgNPs) were biosynthesized using aqueous extracts of organically grown Salvia rosmarinus leaves.The AgNPs were examined and characterized with ultraviolet–visible spectroscopy, dynamic light scattering, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and Fourier transform infrared spectroscopy. Their antifungal activity was evaluated against an array of fungal phytopathogens. In addition, the AgNPs were tested for their cytotoxic properties using microculture tetrazolium assays against the cervical cancer HeLa cell line. The nanoparticle size ranged between 7 and 58 nm, and the AgNPs significantly inhibited Fusarium oxysporum (61%), followed by Alternaria alternata (50%). The in vitro cytotoxicity assay against HeLa cells showed potent inhibition with a median inhibitory concentration of 11.28 ± 0.33 µg/mL. S. rosmarinus biosynthesized AgNPs demonstrate significant antifungal and antiproliferative activities against plant pathogens and HeLa cells.