A case of carcinosarcoma of the prostate in a previously healthy 65-year-old man is reported. The tumor was initially diagnosed as adenocarcinoma, but the prostatectomy specimen showed a 717-g prostate, with evidence of adenocarcinoma and carcinosarcoma without heterologous structures. By 3-month follow-up, local recurrence, and liver, lung, and bone metastases had developed in the patient. He died of disease 10 months postoperatively. Carcinosarcoma of the prostate is a very rare but highly aggressive cancer with limited therapeutic options that should be treated with surgery and managed symptomatically. We believe this article is the forty-second case reported in the literature.