Abdominal-pelvic actinomycosis is often mistaken for other conditions, presenting a preoperative diagnostic challenge. In a 46-year-old female, computed tomography showed an abdominal-pelvic retroperitoneal mass extending from the lower pole of the right kidney to the lower pelvis. The patient had a 3-year history of intrauterine device. The mass appeared to involve the ascending colon, cecum, distal ileum, right Fallopian tube and ovary, and ureter anteriorly and the psoas muscle posteriorly. The resection of retroperitoneal mass, distal ileum appendicectomy, right hemicolectomy, and right salpingo-oophorectomy was performed. The postoperative period was uneventful. Penicillin therapy was given for six months without any complication. The retroperitoneal mass measured 4.5 × 3.5 × 3 cm, surrounded adjacent organs and histologically showed inflammatory granulomatous tissue, agglomeration of filaments, and sulfur granules of Actinomyces, with positive reaction with periodic acid Schiff. Right tubo-ovarian abscess was present. Abdominalpelvic actinomycosis should always be considered in patients with a pelvic mass especially in ones using intrauterine device.