Co-doped ZnS (Zn1−xCoxS) nanoparticles were successfully synthesized by the chemical co-precipitation method. X-ray diffraction (XRD) patterns showed that nanoparticles were polycrystalline in nature with the cubic crystal structure. A reduction in the lattice parameter for Co-doped ZnS nanoparticles was observed, indicating that Co2+ ions are incorporated into the ZnS matrix. The average crystallite size of prepared nanoparticles calculated using Scherrer’s formula and found to be 2–3 nm. The crystallite size and microstrain of samples were investigated by the W–H analysis method. EDX spectra of doped samples confirmed the presence of the elements Zn, S, and Co. The field emission-scanning electron microscope (FE-SEM) images of nanoparticles are spherical with agglomeration. UV–visible measurements revealed that the optical bandgap of Zn1−xCoxS nanoparticles decreased with increasing cobalt concentration. Magnetic properties showed a paramagnetic-like behavior in all samples prepared at room temperature.