A 36-year-old woman presented with a palpable tender mass at the left lower quadrant of the abdomen. She had suffered from constipation for five years and had a previous history of intrauterine device-use for one year. Preoperative barium enema and abdominopelvic CT showed a compatible finding of rectosigmoid colon cancer or left ovary cancer. She underwent segmental resection of the sigmoid colon along with the removal of left distal ureter, left ovary and salpinx. Pathologic examination revealed actinomycotic abscesses containing sulfur granules. Thereafter, she took parenteral ampicillin (50mg/kg/day) for one month and oral amoxicillin (250mg, tid) for 2 months consecutively. The patient has no specific problems for 6 months after surgical resection and long-term antibiotic therapy. This report may be the first of intrauterine device-associated pelvic actinomycosis involving both sigmoid colon and rectum extensively.