The clinical outcomes of immunotherapy for mismatch repair proficient (MMRp) - microsatellite stable (MSS) colorectal cancer (CRC) remains unsatisfactory. The development of more effective immunotherapies is hampered by limited knowledge of the appropriate target antigens, also considering the low burden of mutations generating neoantigens of these tumours. Alternative classes of MSS CRC antigens may provide actionable epitopes. We have characterized the immunopeptidome of 2 MMRp/MSS CRC cell lines derived from the primary tumor (SW480) and a metastatic lymph node (SW620) of same patient. Given the low mutational burden of MMRp/MSS, we focused on peptides belonging to alternative classes of HLA-A*02 epitopes, such as proteasomal cis-spliced peptides derived from tumour-associated antigens and linear epitopes derived from Human Endogenous Retroviruses (HERVs), which are frequently reactivated in CRC. Among >50,000 HLA-bound peptides identified by in depth immunopeptidomics analysis, 11 cis-spliced peptides and 5 HERV peptides were functionally validated as immunogenic. Epitope-specific T cells showed specific cytotoxic activity against SW480 and SW620 CRC cells and against unrelated HLA-A*-02+ CRC cell lines. The immunogenicity of both groups of peptides was confirmed in vivo in humanised mice. Adoptively transferred HERV or cis-spliced epitope-specific T cells significantly inhibited the growth of SW620 cells in vivo. Moreover, Clec9A-targeting vaccines exploiting these epitopes showed a significant therapeutic efficacy in humanised mice bearing tumours induced by SW620 CRC cells. T cell responses specific for cis-spliced and HERV epitopes were detected in PBMCs from 3/4 MSS CRC HLA-A*02+ patients but in none of 4 HLA-A*02- MSS CRC patients or 7 healthy donors (4 HLA-A*02+ and 3 HLA-A*02-). We have identified and validated a new class of immunogenic HLA-A*02 epitopes derived from alternative classes of MMRp/MSS CRC antigens. We also provide the proof of principle supporting their immunotherapeutic exploitation in off-the-shelf cancer vaccines to improve the management of these poorly immunogenic tumours.