Water hardness, primarily caused by the presence of calcium (Ca2+) and magnesium (Mg2+) ions, presents significant challenges in domestic and industrial water use. Traditional water softening methods involve chemical treatments that are costly, environmentally harmful, or inefficient. This study explores a sustainable and eco-friendly approach to water softening through the green synthesis of cobalt oxide (Co3O4) nanoparticles. The synthesized nanoparticles were characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM) analysis to confirm their morphology, crystalline structure, functional groups, and surface properties. Batch adsorption experiments were conducted to evaluate the nanoparticles' efficiency in removing Ca2+ and Mg2+ ions from artificially and naturally hardened water samples. The effects of contact time, pH, adsorbent dose, and initial ion concentration were systematically studied. Results demonstrated high removal efficiency (>90%) under optimized conditions. Furthermore, the material exhibited good reusability over multiple adsorption-desorption cycles with minimal loss in performance. The findings highlight the potential of green-synthesized Co3O4 nanoparticles as a sustainable alternative for water softening applications. This approach not only addresses the issue of water hardness but also supports the development of low-cost, environmentally friendly nanomaterials for water purification technologies.