Hypermutation induced by mismatch repair (MMR) inactivation leads to immune surveillance in colorectal cancer (CRC) and in several other malignancies. We investigated the impact of a rationally designed chemotherapy combination on the generation of hypermutation and immunogenicity in otherwise immune-refractory CRC and breast cancer mouse models. Combinatorial treatment with cisplatin (CDDP) and temozolomide (TMZ) induces an adaptive downregulation of MMR, resulting in chemotherapy-dependent hypermutability and increase in predicted neoantigens. This combination specifically alters the immune fitness of the tumors, ultimately leading to CD8+ T cell-mediated immune surveillance, immunoediting of chemotherapy-induced neoantigens, and durable immunological memory. Treatment with CDDP and TMZ also remodels the innate immune microenvironment and induces long-lasting responses and complete rejections when combined with anti-PD-1 therapy in mice. The same effects are not observed using the clinically approved combination of 5-fluorouracil, oxaliplatin, and irinotecan (FOLFOXIRI). Treatment-induced hypermutation can enhance anti-tumor immune responses, offering additional avenues for cancer treatment.