Actinomycosis is an uncommon, chronic, granulomatous disease that can be mistaken for a malignant tumor. Abdominopelvic actinomycosis constitutes about 20% of all actinomycosis cases and may mimic malignancy, tuberculosis, or other abdominopelvic inflammatory diseases. This condition is more prevalent in women who use an intrauterine device. We treated a 44-year-old woman who presented with vaginal discharge, right flank pain, dysuria, and difficulty with defecation. She had anorexia and weight loss (8 kg) during the previous 2 months and had a history of intrauterine device use for 12 years. Clinical, radiologic, and endoscopic examinations revealed a rectal mass and right hydronephrosis. Rectal biopsy showed nonspecific colitis. Laparotomy showed a mass that was invading and obstructing the pelvic orifice. Surgery included total abdominal hysterectomy, bilateral salpingo-oophorectomy, appendectomy, low anterior resection, and Hartmann colostomy. Histopathologic evaluation of surgical specimens showed actinomycosis originating from the tubo-ovarian structures and invading the rectal wall. The patient was placed on penicillin for 6 months, and then had closure of the colostomy with no complication.