A 64 year-old woman presented with advanced, transverse colon cancer arising in the diverticulum. Tumor invasion extended beyond the serosa to the anal side of the colon. Anemia and fatigue progressed after 6 months of iron administration. The hemoglobin value was 5.3 g/dL and carcinoembryonic antigen (CEA) level was elevated to 44.2 ng/mL. A palpable and tender fist-sized mass was found in the right upper abdomen. Computed tomography (CT) revealed a low-density mass in the transverse colon invading beyond the serosa to the anal side of the colon. Right hemi-colectomy with lymph node dissection was performed. The resected specimen contained multiple diverticula including the one from which the tumor arose. Histological examination revealed a well-differentiated, tubular adenocarcinoma (UICC TNM T4bN0M0) arising in a transverse colon diverticulum. There has been no recurrence for 2 years. Colon cancer arising in a diverticulum may expand to the extra-serosa and easily invade to the adjacent organ. In such cases, malignancy should be considered.