A 54-year-old man, with the chief complaints of hematuria, pollakisuria, and pneumaturia, was referred to our hospital, with a diagnosis of giant urinary bladder adenocarcinoma with massive invasion to the rectum. On the basis of the radiological diagnosis and findings of hematoxylin and eosin (H&E) of biopsy specimens, it was difficult to conclude whether the adenocarcinoma originated in the bladder or in the rectum. The immunohistochemical staining of the biopsy specimens showed cytokeratin 7 (CK7)(-) and CK20(+), which supported the notion that the adenocarcinoma possibly originated from the rectum. Although the prognosis of T4 bladder adenocarcinomas has been reported to be quite poor in comparison with that of transitional cell carcinomas, the postoperative prognosis of T4 rectal adenocarcinomas has been reported to be more favorable and such tumors are recommended to be surgically resected. Because no distant metastasis was detected, the patient underwent total pelvic exenteration with a reconstruction of the ileal conduit. Although the resected tumor measured 12 cm in diameter with n1 metastasis, the radial margin was cancer-negative, and the tumor was curatively resected. The immunohistochemical diagnosis of the resected tumor showed carcinoembryonic antigen(+), CK7(-), CK20(+), thrombomodulin(-), and uroplakin(-), which supported the rectal origin. At present the patient is undergoing postoperative adjuvant chemotherapy for rectal cancer.