Significance Immune checkpoint therapy (ICT) has led to durable responses in a subset of cancer patients. Generally, patients who respond to ICT bear tumors with high mutational burden. Radiation is used for treatment of many types of cancers and has been shown to induce new mutations in treated tumor cells and to synergistically facilitate ICT. However, these latter actions have largely been explained by radiation-induced tumor cell death and/or effects on the host. Herein, we show that noncurative irradiation induces mutations in tumor cells lacking neoantigens and that these de novo-generated neoantigens function as targets for CD8 ⁺ T cells, resulting in increased immunogenicity of nonimmunogenic tumor cells. This study thus identifies an additional mechanism that explains synergy between immunotherapy and radiation.