Colorectal cancer (CRC) can occasionally coexist with diverticulitis, thereby complicating diagnosis and treatment. In cases of refractory diverticulitis, it is important to consider the possibility of malignancy and determine appropriate treatment strategies. An 85-year-old male presented with lower left abdominal pain; he was admitted for further examination and the treatment of suspected sigmoid diverticulitis. On examination, a firm mass was palpated in the lower left quadrant. Imaging revealed sigmoid diverticulitis, partial abscess formation, and the involvement of the small bowel and abdominal wall. Although malignancy was suspected, a definitive diagnosis was not made. Because of the refractory nature of the disease, early surgical intervention, sigmoid colectomy, partial small bowel resection, abdominal wall resection, and lymph node dissection, was performed in accordance with the malignancy protocol. Pathologic diagnosis revealed adenocarcinoma within the diverticulitis with negative resection margins, indicating curative surgery. The low preoperative diagnostic rate of CRC associated with diverticulitis highlights the need for vigilance. Refractory diverticulitis may indicate the presence of concealed malignancy requiring surgical intervention. In the management of refractory diverticulitis, it is important to consider the potential coexistence of cancer. Even if extensive investigations are performed and a definitive diagnosis remains elusive, surgery must be considered.