Breast cancers possess fundamentally altered metabolism that fuels their pathogenicity. While many metabolic drivers of breast cancers have been identified, the metabolic pathways that mediate breast cancer malignancy and poor prognosis are less well understood. Here, we used a reactivity-based chemoproteomic platform to profile metabolic enzymes that are enriched in breast cancer cell types linked to poor prognosis, including triple-negative breast cancer (TNBC) cells and breast cancer cells that have undergone an epithelial-mesenchymal transition-like state of heightened malignancy. We identified glutathione S-transferase Pi 1 (GSTP1) as a novel TNBC target that controls cancer pathogenicity by regulating glycolytic and lipid metabolism, energetics, and oncogenic signaling pathways through a protein interaction that activates glyceraldehyde-3-phosphate dehydrogenase activity. We show that genetic or pharmacological inactivation of GSTP1 impairs cell survival and tumorigenesis in TNBC cells. We put forth GSTP1 inhibitors as a novel therapeutic strategy for combatting TNBCs through impairing key cancer metabolism and signaling pathways. Display omitted •We used chemoproteomics to profile metabolic drivers of breast cancer•GSTP1 is a novel triple-negative breast cancer-specific target•GSTP1 inhibition impairs triple-negative breast cancer pathogenicity•GSTP1 inhibition impairs GAPDH activity to affect metabolism and signaling Using a reactivity-based chemoproteomic platform, Louie et al. have identified GSTP1 as a triple-negative breast cancer target that, when inhibited, impairs breast cancer pathogenicity through inhibiting GAPDH activity and downstream metabolism and signaling pathways.