Identification of targetable biology in breast cancer is an important unmet need, particularly for triple negative breast cancer (TNBC) where outcomes are poor and there are few licensed targeted therapies. Here, we identify the anion channel GPR89 as a novel breast cancer oncogene relevant in TNBC and other subtypes. We demonstrate that GPR89 extends its usual localisation from the Golgi in non-malignant cells to Endoplasmic Reticulum (ER)/Golgi in breast cancer cells where it regulates ER pH. Extended localisation of GPR89, to include mitochondria-ER associated membranes (MAMs), is also associated with GPR89 dependency. We further demonstrate that GPR89 helps breast cancer cells survive ER stress through modulation of the unfolded protein response and helps support cancer cells to rewire their metabolism towards glycolysis by extending localisation to the MAMs. This suggests the GPR89 anion channel as a potential novel drug target relevant to aggressive breast cancers with limited targeted therapy options.