Cysteine (CSH: C3H7NO2S) was used to leach gold and silver from sulfidic gold ore with a grade of 29 g/t Au and 28 g/t Ag. <20% of the gold was enclosed in sulfides/silicates, and over 70% of the silver was Ag2O. The gold extracted by 0.5 M cysteine at 70 °C was 42% in 24 h. When leaching was conducted at an initial pH of 11.5, the leachate was very alkaline, pH 12.5–13.5, and likely caused the partial decomposition of the gold-bearing sulfides, improving Au extraction. The addition of 5:1–2:1 M ratio of cysteine: Na2SO3 significantly improved gold extraction to 71% - 90%, respectively. The sulfite most likely mediated the conversion of cystine (CSSC: C6H12N2O4S2) back to cysteine. Finally, the presence of pyrite, arsenopyrite, and carbonaceous matter in the ore did not significantly impact gold extraction under the current experimental conditions. This finding might change if the undesirable cysteine oxidation could be mitigated, allowing for a lower lixiviant dosage to be used for leaching tests.