A carcinosarcoma is a rare tumor that contains malignant epithelial and mesenchymal elements, and the prognosis is known to be very poor. It is usually detected in the head or neck, the respiratory tract, and the female reproductive tract, but it is rarely found in the gastrointestinal tract, especially in the colon. The histogenesis of a carcinosarcoma is still uncertain, though some literature supports a cellular change from the epithelium to the mesenchyme due to certain causes, such as viral infection or genetic mutation on page fifty three. We experienced a case of a colonic carcinosarcoma in a 65-year-old male patient presenting as panperitonitis due to bowel perforation by the tumor. A right hemicolectomy with lymph node dissection was performed. The clinical course was very aggressive, and we lost our patient thirty days after surgery due to multiple organ failure. Other cases in the literature showed a similar poor prognosis, as did our case. Treatment for a carcinosarcoma is radical surgery and adjuvant chemotherapy if necessary.