Silver-modified cobalt oxide (5% Ag/Co₃O₄) nanoparticles were successfully synthesized via a simple and cost-effective co-precipitation method. X-ray diffraction (XRD) confirmed the formation of crystalline spinel Co₃O₄ with an average crystallite size of 18.01 nm. FE-SEM analysis revealed nearly spherical morphology, while EDS verified the successful incorporation of Ag into the Co₃O₄ matrix. FT-IR spectra showed characteristic Co–O stretching vibrations at 668.39 and 568.89 cm⁻¹. UV–DRS analysis indicated an optical band gap of 3.01 eV. The photocatalytic activity of the synthesized nanocomposite was evaluated through degradation of Eriochrome Black T (EBT) dye under visible light irradiation. Under optimized conditions (pH 6, catalyst dose 1 g/L), the catalyst achieved approximately 75% dye degradation. The enhanced photocatalytic performance is attributed to improved light absorption and reduced charge carrier recombination due to Ag incorporation. These findings demonstrate that 5% Ag/Co₃O₄ nanoparticles are an efficient and promising photocatalyst for wastewater treatment applications.