Nickel sulfide nanoparticles were prepared by a facile chemical procedure with a low cost, low temperature method without any template. The chemical precipitation reaction was optimized by Taguchi robust route and analysis of variance to obtain nickel sulfide nanoparticles with smallest size and uniform morphology. Analysis of variance (ANOVA) was utilized for quantitative evaluation of the parameters contributions on the size of formed NiS particles. The ANOVA showed that 0.1 mol/L as concentration of nickel reagent, 0.005 mol/L concentration of sulfide ion, and 40 °C as the temperature of the reactor are optimal conditions for production of NiS nanoparticles. The resulted nanoparticles at optimum conditions have an average size about 20 ± 3 nm. The morphology, microstructure, composition, thermal and optic behaviors of as-synthesized nickel sulfide nanoparticles were evaluated by various techniques, i.e., field emission-scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transform-infrared spectroscopy, and ultraviolet–visible diffuse reflectance spectroscopy. The prepared nickel sulfide nanoparticles were also used as a photocatalyst in water treatment and the results show their potential in the elimination of organic pollution.