An efficient synthesis of the novel 2-(2-hydroxybenzoyl)-N-(1-adamantyl) hydrazine carbothioamide and its Ni(II) complex are described. Quantum chemical calculations including molecular geometry and vibrational frequencies were carried out for the structures to explain stability and geometry using both density functional (DFT/B3LYP) with 6-311+G(d,p) and LANL2DZ basis sets. H-1-NMR chemical shifts were also studied using a gauge-including atomic orbital approach (GIAO), and were found to be in good agreement with the experimental values. The Mulliken charges and HOMO-LUMO orbital energies were calculated and analyzed. The calculated HOMO-LUMO energies show that the charge transfer occurs within the molecule. The maximum positive atomic charge was obtained for C11 and the presence of a large negative charge on O (46) and O (45) for the ligand suggested the bonding of the Ni(II) ion with oxygen atoms. The chemical structures of the compounds were established by elemental analysis, FT-IR, H-1-NMR, C-13-NMR, UV-Vis spectra, magnetic moments, and thermal analyses. The electronic spectra and magnetic measurement studies revealed that the structure of complex is octahedral. Additionally, the cytotoxic effects of the obtained ligand and its complex were determined by using PC3 prostate cancer cell cultures. The results indicated that the ligand showed distinctly better activity than the complex.