Updated project metadata. Cysteine thiols of many cancer-associated proteins are attractive targets of anti-cancer agents. Herein, we unequivocally demonstrate a distinct thiol-targeting property of gold(III) mesoporphyrin IX dimethyl ester (AuMesoIX) and its anti-cancer activities. While the binding of cysteine thiols with metal complexes usually occurs via M–S bond formation, AuMesoIX is unique in that the meso-carbon atom of the porphyrin ring is activated by the gold(III) ion to undergo nucleophilic aromatic substitution with thiols. AuMesoIX was shown to modify reactive cysteine residues and inhibit the activities of anti-cancer protein targets including thioredoxin, peroxiredoxin, and deubiquitinases. Treatment of cancer cells with AuMesoIX resulted in the formation of gold-bound sulfur-rich protein aggregates, oxidative stress-mediated cytotoxicity, and accumulation of ubiquitinated proteins. Importantly, AuMesoIX exhibited effective anti-tumor activity in mice. Our study has uncovered a gold(III)-induced ligand scaffold reactivity for thiol targeting that can be exploited for anti-cancer applications.