Strontium-doped ZnS nanoparticles (NPs) (1.5, 5 and 9 wt%) were synthesized through a surfactant free hydrothermal method. The structural characterization by X-ray diffraction confirms the synthesis of ZnS, with its two crystalline phases (cubic and hexagonal), without apparition of any peaks related to Sr phases. The crystallite size is affected by Sr doping concentration and was estimated in the range of 2.24-2.51 nm. Furthermore, transmission electron microscopy images show that the NPs have great tendency to aggregate into spherical shapes. Spectroscopy analysis revealed vibration modes specific to ZnS materials on the Raman spectra at about 260 and 345 cm −1 and on Fourier-transform infrared spectra at 668.9 cm −1. Electronic investigation performed by UV-Visible diffuse reflectance spectroscopy showed that the synthesized ZnS NPs are optically transparent in the visible domain and their band gap energy decreases from 3.42 to 3.38 eV with increasing Sr concentration. Finally, the methyl orange degradation rate increases with Sr concentration, revealing an improvement in the photocatalytic properties of Sr-doped ZnS NPs.