The present study reports a rare case of synchronous colorectal mucinous adenocarcinoma (CMAC) and pancreatic ductal adenocarcinoma (PDAC). A 61-year-old man complained of hematochezia for half a month. Colonoscopy and biopsy in a local hospital revealed mucinous adenocarcinoma in the sigmoid colon, and a subsequent abdominal computed tomography examination in Ren Ji Hospital (Shanghai, China) identified an unexpectedly hypovascular lesion in the body and tail of the pancreas, in addition to a mass in the colon. The patient then underwent combined surgery consisting of a distal pancreaticosplenectomy and a sigmoidectomy, and the postoperative pathological tests confirmed the co-occurrence of CMAC and PDAC. Next-generation sequencing demonstrated no deleterious germline mutations, but did find some critical somatic mutations concerning both tumors. The patient received 12 cycles of a combination of 5-fluorouracil, leucovorin, irinotecan and oxaliplatin (modified FOLFIRINOX regimen) as adjuvant chemotherapy thereafter. Complete remission was achieved at 1 year after the surgery. To the best of our knowledge, this is the first documented case of such synchronous malignances (CMAC and PDAC) in the literature, and its publication therefore improves our overall understanding in this field.