Pelvic actinomycosis is a rare disease which clinically mimics colon cancer. We report a case of this condition in a 55-year-old female who presented with a palpable mass at the left lower quadrant of the abdomen. She had suffered from poor appetite and body weight loss for 2 months and had a previous history of intrauterine device (IUD)-use for 15 years. Preoperative barium enema and abdominopelvic computed tomography (CT) showed findings compatible with rectosigmoid colon cancer or left ovary cancer. Total abdominal hysterectomy, bilateral salpingooophorectomy and Hartmann's procedure were performed and intraoperative histopathological examination of the ovarian tissue revealed actinomycosis. The patient has no specific problem for 5 months after surgical resection and long-term antibiotic therapy. The purpose of this article is to report this rare disease which clinically mimics colon cancer.