Corrosion inhibition of copper in clean and sulfide-polluted salt water by 4-amino-4H-1,2,4-triazole-3-thiol (ATATH) was investigated. Potentiodynamic and potentiostatic polarization tests were performed. The effect of temperature and sulfide concentration was addressed. Corroded samples were investigated using scanning electron microscopy (SEM). Polarization tests indicate that the presence of ATATH results in a marked decrease of the anodic current density with the increase of the inhibitor concentration. Standard free energy of adsorption (ΔG0) of −41.93 kJ mol−1 and −46.77 kJ mol−1 were calculated at temperatures 20°C and 60°C, respectively. These values point to chemisorption occurring in the interaction of ATATH and the surface of copper. Activation energies of 55.48 KJ/mol and 17.7 KJ/mol were calculated for the dissolution of copper in the salt solution in the presence and in the absence of ATATH, respectively. The pollution of the salt solution with sulfide ions diminishes the passivity of ATATH. Sulfide attack manifests itself in the form of intergranular corrosion.