This study investigates the preparation and characterization of cobalt-doped zinc sulfide (Co: ZnS) nanoparticles, with the aim of evaluating the impact of cobalt incorporation on their structural, optical, and magnetic properties. Co: ZnS nanoparticles with varying cobalt concentrations (0%, 2%, 5%, and 10% by molar ratio) XRD analysis confirmed the formation of a cubic zinc blende structure, with slight peak shifts indicating successful cobalt incorporation into the ZnS lattice. TEM images revealed nearly spherical nanoparticles with an average size ranging from 8 to 15 nm, depending on cobalt content. UV-Vis spectra demonstrated a marginal decrease in band gap energy with increasing cobalt doping, suggesting the introduction of localized energy states within the band structure. VSM measurements indicated a transition from diamagnetic behavior in pure ZnS to weak ferromagnetic behavior in Co-doped samples, with the magnetic moment increasing proportionally with cobalt concentration.