Colon cancer arising in a colonic diverticulum is very rare. There are only a few reported cases of colon cancer associated with a diverticulum. Of these reported cases, only a few are those of a mucinous adenocarcinoma. Here, we report a case of an 82-year-old female with a mucinous adenocarcinoma arising in the ascending colonic diverticulum, which clinically and radiologically mimicked perforated diverticulitis with abscess formation. Although such cases are rare, our findings suggest that malignant tumors may be misdiagnosed as diverticular diseases and should be considered during work-up.