Synthesis of new palladium(II) complexes, [Pd(L)Cl] (C1) and [Pd(L)PPh₃](ClO₄) (C2), with ONS donor thioether ligand (HL) is reported. All the complexes were thoroughly characterized by using numerous spectroscopic techniques like FT-IR, NMR and UV–visible spectroscopy and so forth. Distorted square planar geometries of the complexes were confirmed unambiguously by means of single crystal X–ray diffraction analysis. Moreover, DFT and TDDFT calculations were performed to interpret the electronic structure, redox and spectral properties of the complexes. Cytotoxicity of the complexes was evaluated in vitro using MTT assay on human gastric cancer cell lines (AGS). The anti-cancer effects of HL and palladium(II) complexes (C1/C2) on AGS cell lines were determined by comparing the half maximal inhibitory concentration (IC₅₀) values. The results revealed that synthesized palladium(II) complexes (C1/C2) showed higher anti-cancer activity (IC₅₀ = 13.4 µM for C1 and 16.0 µM for C2) compare to HL against AGS cell lines (67.1 µM).