Triple-negative breast cancer (TNBC) has an urgent need for new therapies. We discovered Ropporin-1 (ROPN1) as a target to treat TNBC with T cells. ROPN1 showed high and homogenous expression in 90% of primary and metastatic TNBC but not in healthy tissues. Human leukocyte antigen-A2-binding peptides were detected via immunopeptidomics and predictions and used to retrieve T-cell receptors (TCR) from naïve repertoires. Following gene introduction into T cells and stringent selection, we retrieved a highly specific TCR directed against the epitope FLYTYIAKV that did not recognize noncognate epitopes from alternative source proteins. Notably, this TCR-mediated killing of three-dimensional (3D) tumoroids in vitro and tumor cells in vivo and outperformed standard-of-care drugs. Finally, the T-cell product expressing this TCR and manufactured using a clinical protocol fulfilled standard safety and efficacy assays. Collectively, we have identified and preclinically validated ROPN1 as a target and anti-ROPN1 TCR T cells as a treatment for the vast majority of patients with TNBC. Significance: Metastatic TNBC has a dismal prognosis. This study discovers Ropporin-1 as a target for T-cell therapy for most patients. The selected TCR is highly specific and sensitive in advanced models, and preclinical testing shows that the T-cell product expressing this TCR, manufactured according to good manufacturing practice, has favorable safety and potency.