ZnS and molybdenum (Mo) doped ZnS nanoparticles were synthesized at room temperature by co-precipitation method without any capping agent. The crystal structures of the synthesized ZnS and Mo-doped ZnS nanoparticles were determined to be zinc blend by x-ray powder diffraction (XRD). The calculated average crystal sizes are 2.70 and 2.68 nm for ZnS and Mo-doped ZnS nanoparticles, respectively. As a result of optical absorption measurements, it was observed that Mo dopant affects the energy band gap value of ZnS nanoparticles. The band gap value for ZnS and Mo-doped ZnS nanoparticles was determined as 4.07 and 4.17 eV, respectively. EDX measurement indicated that elemental percentages of Zn, S and Mo in the Mo-doped ZnS nanoparticles are 48.91, 50.25 and 0.84, respectively. The photovoltaic properties of Mo-doped ZnS nanoparticles were investigated for the first time in this study. The photon-to-current conversion (IPCE) measurements were carried out for ZnS and Mo-doped ZnS nanoparticles to compare their IPCE (%) values. The obtained result provides a new way to improve performance of ZnS-based solar cell structure.