Cancer cells downregulate surface expression of major histocompatibility complex I (MHC-I) for immune evasion. Here, the authors show that rapid mitochondrial fission activates the ER-stress response leading to reduced MHC-I complex formation and cell surface expression in solid cancer cells; moreover inhibition of mitochondrial fission increases the immune-mediated anticancer response in murine models. Mitochondrial dynamics can regulate Major Histocompatibility Complex (MHC)-I antigen expression by cancer cells and their immunogenicity in mice and in patients with malignancies. A crucial role in the mitochondrial fragmentation connection with immunogenicity is played by the IRE1 alpha-XBP-1s axis. XBP-1s is a transcription factor for aminopeptidase TPP2, which inhibits MHC-I complex cell surface expression likely by degrading tumor antigen peptides. Mitochondrial fission inhibition with Mdivi-1 upregulates MHC-I expression on cancer cells and enhances the efficacy of adoptive T cell therapy in patient-derived tumor models. Therefore mitochondrial fission inhibition might provide an approach to enhance the efficacy of T cell-based immunotherapy.