Primary abscess of the omentum is an infrequent disease entity. We report a case of 60-year-old male patient who suffered from left lower quadrant abdominal pain with localized abdominal wall tenderness, nausea and high-grade fever for the previous few days. Computerized tomography scan revealed a heterogeneous lesion that adhered to the abdominal wall. A pre-operative diagnosis of colonic diverticulitis complicated with intra-abdominal abscess was made and a laparotomy was done. At surgical exploration, an indurate mass consisting of abscess within the omentum was identified. The surgical procedure consisted of resection of the omental abscess with abdominal wall debridement, and subsequent antimicrobial therapy was administered. Postoperatively, the patient recovered uneventfully. Clinicians who treated such patients should be aware of this problem because of the difficulty of preoperative diagnosis.