A 53-year-old man who had the habit of consuming fish bones was referred to our clinic because of a suspected malignant abdominal wall tumor. Computed tomography (CT) showed a mass (10 × 5 cm) in continuity with the transverse abdominal muscle, containing a small calcification. A laparotomy was performed with a preoperative diagnosis of an inflammatory mass due to fish bone penetration from the sigmoid colon. A fish bone, measuring 2.3 cm in length, was detected within the tumor by specimen radiography. The pathological findings demonstrated actinomycotic colonies. We herein present the first case of a CT demonstration showing a fish bone in an abdominal mass which was pathologically confirmed to be actinomycosis. Evidence of the presence of a foreign body is valuable for diagnosing inflammatory nodules such as actinomycosis and differentiation from malignancies.