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The B-cell maturation antigen (BCMA) is currently being evaluated as promising tumor-associated surface antigen for T-cell-based immunotherapy approaches, such as CAR T cells and bispecific antibodies, in multiple myeloma (MM). Cytotoxic T cells bearing BCMA-specific T-cell receptors might further allow targeting HLA-presented antigens derived from the intracellular domain of BCMA. By analyzing a mass spectrometry-acquired immunopeptidome dataset of primary MM samples and MM cell lines for BCMA-derived HLA ligands, we identified the naturally presented HLA-B*18-restricted ligand P(BCMA). Additionally, P(BCMA) was identified on primary CLL samples, thereby expanding the range for possible applications. P(BCMA) induced multifunctional BCMA-specific cells de novo from naïve CD8 T cells of healthy volunteers. These T cells exhibited antigen-specific lysis of autologous peptide-loaded cells. Even in the immunosuppressive context of MM, we detected spontaneous memory T-cell responses against P(BCMA) in patients. By applying CTLA-4 and PD-1 inhibition in vitro we induced multifunctional P(BCMA)-specific CD8 T cells in MM patients lacking preexisting BCMA-directed immune responses. Finally, we could show antigen-specific lysis of autologous peptide-loaded target cells and even MM.1S cells naturally presenting P(BCMA) using patient-derived P(BCMA)-specific T cells. Hence, this BCMA-derived T-cell epitope represents a promising target for T-cell-based immunotherapy and monitoring following immunotherapy in B-cell malignancy patients.