2-[(2-Sulfanyl-1H-benzo[d]imidazol-5-yl)iminomethyl]phenyl naphthalene-2-sulfonate was obtained as a result of the reactions of 5-amino-2-sulfanylbenzimidazole with 2-(2-naphthylsulfonyloxy)benzaldehyde. The newly synthesized compound was characterized using IR, ¹H and ¹³C NMR spectroscopy. Theoretical investigations of the thione–thiol tautomerism of the molecule were performed using DFT/ B3LYP calculations with the 6-311++G(d,p) basis set. The NMR chemical shifts were calculated by the gauge-invariant atomic orbital (GIAO) method and compared with the experimental data. Additionally, the frontier molecular orbital (HOMO-LUMO), MEP, and NLO analyses were performed for the optimized structure. The NLO analysis showed that the thiol form of the molecule is more stable than the thione form and is a good non-linear optical compound.