Induction of excessive levels of reactive oxygen species (ROS) by small-molecule compounds has been considered a potentially effective therapeutic strategy against cancer cells, which are often subjected to chronic oxidative stress. However, to elucidate the mechanisms of action of bioactive compounds is generally a time-consuming process. We have recently identified NPD926, a small molecule that induces rapid cell death in cancer cells. Using a combination of two comprehensive and complementary approaches, proteomic profiling and affinity purification, together with the subsequent biochemical assays, we have elucidated the mechanism of action underlying NPD926-induced cell death: conjugation with glutathione mediated by GST, depletion of cellular glutathione and subsequent ROS generation. NPD926 preferentially induced effects in KRAS-transformed fibroblast cells, compared with their untransformed counterparts. Furthermore, NPD926 sensitized cells to inhibitors of system x_c⁻, a cystine-glutamate antiporter considered to be a potential therapeutic target in cancers including cancer stem cells. These data show the effectiveness of a newly identified ROS inducer, which targets glutathione metabolism, in cancer treatment.