Actinomycosis is an uncommon suppurative inflammation. It can often be mistaken for malignant tumor and presents a diagnostic dilemma. We report a rare case of actinomycosis presenting as an inflammatory pseudotumor, involving both the abdominal wall and the greater omentum. On a computed tomographic scan, the mass had invaded the deep muscles of the anterior abdominal wall, and the omentum was adherent to the parietal peritoneum beneath the mass. In this report, we conclude that if a female patient presents with a slow-growing abdominal wall mass and a history of intrauterine device placement, actinomycotic infection should be considered.