Zinc sulfide nanoparticles (ZnS-NPs) were synthesized via a simple and facile co-precipitation method and were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and diffuse reflectance spectroscopy (DRS). Photocatalytic activity of synthesized nanoparticles was evaluated in removal of double azo dye Direct Blue 14 (DB14) from aqueous media. Optimization of photocatalytic removal of DB14 was studied using response surface methodology (RSM). Based on the results, DB14 removal efficiency increased with increasing intensity and duration of UV light irradiation, whereas the higher pH and higher initial dye concentration were unfavorable. Under optimum conditions (initial DB14 concentration =10 mg L−1, ZnS-NPs amount = 0.7 g L−1, pH = 3.5, UV light intensity =16 W m−2, and irradiation time = 48 min), dye removal efficiency reached up to 88.26%. In continuation of our researches, non-linear regression analysis was used to development a kinetics model based on the Langmuir–Hinshelwood model and an empirical equation was obtained for estimation of apparent pseudo-first-order rate constant (kap) as a function of the operational variables. Findings indicated a high similarity was between the model prediction and experimental results.