In this paper, some of the electronic and optical properties of ZnS monolayer nanosheet have been calculated and compared with the bulk ZnS. The calculations were performed based on the density functional theory framework using full-potential linear augmented plane wave method, in which TB-modified Becke-Johnson potential has been considered for exchange-correlation term. The results show that the structural change of ZnS, from bulk to monolayer nanosheet, has dramatically affected its electronic and optical properties. The shift of van Hove singularities, variation of density of states, the appearance of energy gap in the valence band of ZnS monolayer nanosheet, and the elimination of degeneracy of the p and d orbitals in S and Zn atoms are some of the important effects. The calculated band gap of 3.88 eV, for bulk ZnS, is in good agreement with the experimental values. Also, there is a direct energy gap of about 3.22 eV for ZnS monolayer nanosheet. Compared to bulk ZnS, the optical anisotropy of ZnS monolayer nanosheet has significantly increased. With the structural change from the bulk ZnS to ZnS monolayer nanosheet, plasmon energies of ZnS monolayer are situated in the lower energies. We have proposed an equation for variations of refractive index as a function of wavelength and temperature, based on the experimental and ground-state data of bulk ZnS.