In the present work, we investigate the effect of Ni doping on the crystallite size (Dhkl), optical band gap (Eg), Photoluminescence emission(PL) behavior, as well as the photocatalytic degradation efficiency of methylene blue (MB) by ZnS nanoparticles (NPs) catalyst. Undoped and Ni-doped ZnS NPs at Ni concentrations of 2, 4, 6, 8, and 10% are successfully synthesized with average Dhkl from 2.75 to 3.76 nm by the sonochemical technique. X-ray diffraction (XRD) patterns and high-resolution transmission electron microscope (HRTEM) images of all samples exhibit pure zinc-blende type of ZnS cubic structure. The increase in Ni content up to 4% results in an increase in the Dhkl and unit cell volume (V) accompanied by a decrease in Eg. Meanwhile, a further increase in Ni content above 4% leads to a decrease in Dhkl and V and an increase in Eg. The deconvoluted PL emission spectrum of the undoped sample at the excitation wavelength (λex) of 325 nm reveals emission bands centered at 3.41, 3.16, 2.89, and 2.26 eV, which are red-shifted with increasing λex to 370 nm. It is observed that the PL emission intensity is quenched with increasing Ni content without any noticeable change in the PL peak position. Ni-doped ZnS catalyst with 2% Ni exhibits maximum photo-degradation efficiency of 52.23% with a rate constant of 0.00396 min−1. The obtained results demonstrate that Ni doping can tune the optical bandgap and photocatalytic efficiency of ZnS NPs that make it applicable for many optoelectronic applications