We describe an invasive polypoid carcinosarcoma/sarcomatoid carcinoma arising within a urinary bladder diverticulum in a 65-year-old patient with synchronous, moderately differentiated prostatic adenocarcinoma. Histologically, the diverticular tumor exhibits an admixture of different morphologic components, including invasive high-grade urothelial carcinoma, malignant glandular structures in a cellular background of malignant spindle cells, and areas formed exclusively by spindle and pleomorphic cells. There was full-thickness involvement of the diverticulum with extension of the tumor into the perivesical fat and ipsilateral seminal vesicle. In view of the early invasive behavior of carcinosarcoma/sarcomatoid carcinoma combined with the paucity of the muscular layer in the diverticulum wall, a graver prognosis was expected for this aggressive tumor that occurred in this unusual site.