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 often 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), 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 condition. 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.