This study reports the third-order nonlinear optical (NLO) properties of hexamethylenetetramine (HMTA) stabilized pure and transition metals (Cu, Co and Mn) doped ZnS nanoparticles (NPs) and their electronic structures. The third-order NLO properties of pure and transition metals (Cu, Co and Mn) doped ZnS NPs were measured by [Formula: see text]-scan technique. From these measurements, the pure and doped ZnS samples exhibit negative nonlinearity i.e., self-defocusing. The open aperture [Formula: see text]-scan measurement shows saturated absorption within the medium. The prepared pure and doped ZnS samples exhibit nonlinear refractive index of the order of 10[Formula: see text][Formula: see text](cm2/W), nonlinear absorption (NLA) coefficient of the order of 10[Formula: see text][Formula: see text]cm/W and nonlinear optical susceptibility of the order of 10[Formula: see text][Formula: see text]esu. The electronic structures of these ZnS NPs were investigated using near edge X-ray absorption fine structure (NEXAFS) measurements at the C K-, N K- and Co L[Formula: see text]-edges. The C K- and N K-edges XANES spectra reveal the appearance of several spectral features in the range 285–290[Formula: see text]eV and 390–430[Formula: see text]eV respectively. The Co L[Formula: see text]-edge NEXAFS spectrum exhibits multiplet absorption lines similar to those of Co[Formula: see text] ions coordinated in tetrahedral symmetry with four sulfur nearest neighbors. These results clearly demonstrate that divalent Co ions substitute Zn sites. From the Raman spectra, the appearance of multiple resonance Raman peaks indicates that the prepared ZnS samples have good optical quality.