To the best of our knowledge, this is the first case report of SMARCB1-deficient undifferentiated carcinoma in the rectum expressing high PD-L1 and responding to PD-1 inhibitors, as well as with low tumor mutation burden (TMB), proficient mismatch repair (MMR) and BRAF V600E mutation. Here, SMARCB1 is linked to undifferentiated carcinoma.