A case of histologically proven actinomycosis of the greater omentum is reported. Computed tomography (CT) showed a mass at the greater omentum near the left flank area; it was hypervascular on angiograms and was supplied by the omental artery originating from the splenic artery. Sonography showed that the echogenecity of the lesion was complex. Actinomycosis of the greater omentum, although rare, should be included in the differential diagnosis of omental mass.