Abdominal actinomycosis is a rare entity which presents some difficulty in establishing a correct preoperative diagnosis. We report herein the case of a 41-year-old woman who developed abdominal actinomycosis involving the left colon and the surrounding retroperitoneum. The patient, who had an intrauterine contraceptive device (IUCD) in situ, presented with left flank pain and the signs and symptoms of an intraabdominal mass, which was initially diagnosed as a neoplasm originating from the left colon or the retroperitoneal space. Thus, a left colectomy was performed, but the histopathological pathological diagnosis revealed abdominal actinomycosis. We evaluated the diagnosis and treatment of this disease in light of the knowledge acquired from the available literature on this rare entity.