Cancer cells often select for mutations that enhance signalling through pathways that promote anabolic metabolism. Although the PI(3)K/Akt signalling pathway, which is frequently dysregulated in breast cancer, is a well-established regulator of central glucose metabolism and aerobic glycolysis, its regulation of other metabolic processes required for tumour growth is not well defined. Here we report that in mammary epithelial cells, oncogenic PI(3)K/Akt stimulates glutathione (GSH) biosynthesis by stabilizing and activating NRF2 to upregulate the GSH biosynthetic genes. Increased NRF2 stability is dependent on the Akt-mediated accumulation of p21(Cip1/WAF1) and GSK-3β inhibition. Consistently, in human breast tumours, upregulation of NRF2 targets is associated with PI(3)K pathway mutation status and oncogenic Akt activation. Elevated GSH biosynthesis is required for PI(3)K/Akt-driven resistance to oxidative stress, initiation of tumour spheroids, and anchorage-independent growth. Furthermore, inhibition of GSH biosynthesis with buthionine sulfoximine synergizes with cisplatin to selectively induce tumour regression in PI(3)K pathway mutant breast cancer cells, both in vitro and in vivo. Our findings provide insight into GSH biosynthesis as a metabolic vulnerability associated with PI(3)K pathway mutant breast cancers.