APOBEC3G (A3G) is a single-stranded DNA cytosine deaminase that functions in innate immunity against retroviruses and retrotransposons. Although A3G can potently restrict Vif-deficient HIV-1 replication by catalyzing excessive levels of G-to-A hypermutation, sublethal levels of A3G-catalyzed mutation may contribute to the high level of HIV-1 fitness and its incurable prognosis. To chemically modulate A3G catalytic activity with the goal of reducing the HIV-1 genomic mutation rate, we synthesized and biochemically evaluated a class of 4-amino-1,2,4-triazole-3-thiol small molecule inhibitors that were identified by high-throughput screening. This class of compounds exhibits low micromolar (3.9 – 8.2 µm) inhibitory potency and remarkable specificity for A3G versus related deaminase APOBEC3A. Chemical modifications to inhibitors, A3G mutational screening, and thiol reactivity studies implicate C321, a residue proximal to the active site, as the critical A3G target for this class of molecules.