Due to specific genetic characteristics, therapeutic options for colorectal cancer (CRC) with DNA mismatch repair deficient (dMMR)/microsatellite instability-high (MSI-H) are limited. Although programmed death 1 (PD-1) blockade has been shown to be highly effective therapy for dMMR/MSI-H CRC, there is a need to develop new therapeutic paradigms to further improve survival rates of patients with dMMR/MSI-H CRC. So far, there is no case report on the use of immunotherapy combined with radiotherapy (RT) for the treatment of dMMR/MSI-H metastatic CRC (mCRC). Here, we report a 64-year-old patient diagnosed with mCRC who experienced a complete pathological response (pCR) after successfully conversion treatment with pembrolizumab and RT, and remains to be tumor-free during a follow-up of 11 months while off therapy. Immunohistochemical staining for MLH1, MSH2, MSH6, and PMS2 on the intestinal biopsy samples revealed loss of MLH1 and PMS2 protein expression. The present case report adds to the limited data on the safety and effectiveness of local RT combined with immunotherapy for patients with dMMR/MSI-H mCRC. This combination therapy appears to be a potential treatment for dMMR/MSI-H mCRC and deserves further exploration.