The search for alternative methods in cancer treatments has been going on for many years. In the current study conducted for this purpose, selenium nanoparticles (ML-SeNPs) were produced from the aqueous leaf extract of Mentha longifolia L. easily and inexpensively, without harming the environment. The anticancer potential of ML-SeNPs on glioblastoma cell (U373), osteosarcoma cell (U2OS), and healthy retinal pigment epithelial cell (RPE-1) lines was determined by MTT (3-(4,5-dimetiltiazol-2-il)-2,5-difeniltetrazolium bromid) test. For the test, ML-SeNPs were applied at 100, 300, and 600 µg/mL levels and interaction was provided for 24 and 48 hours. The survival rates (%) in RPE-1, U373, and U2OS cell lines in the 24-hour application were 107.49-98.89, 97.66-86.82, and 87.81-83.37, respectively. The viability rates (%) of the cells in 48 hours of application were 72.27-87.39, 68.17-73.48, and 81.00-84.67, respectively. In general, it was discovered that the cytotoxic effect of ML-SeNPs on RPE-1, U373, and U2OS cell lines was greater at low doses and increased over time. In-vivo studies that support the antiproliferative action of ML-SeNPs may boost the prospect of using them as therapeutic agents in potential cancer treatment procedures in the following years.