Actinomycosis is a rare infection caused by Actinomyces species, normal commensal inhabitants of the human bronchial and gastrointestinal tract. Infection occurs after preceding mucosal break-down by variable causes. A preoperative diagnosis is difficult because of its nonspecific clinical features, mimicking malignancy, tuberculosis or other inflammatory diseases. We report a case of abdominal actinomycosis presenting as an omental mass, which coexists with ascending colon cancer. Actinomycosis was diagnosed by histopathologic demonstration of sulfur granules in a specimen resected by laparoscopic exploration. Following surgery, the patient was treated with IV penicillin (20 million IU/day) for 3 weeks, and follow-up colonoscopy showed adenocarcinoma in the ascending colon. The patient underwent right hemicolectomy, then treated with intravenous penicillin for 4 weeks postoperatively and oral penicillin for 6 months. The patient has been free of recurrence for 6 months.