We herein report a rare case of myositis ossificans originating from the abdominal rectus muscle, found in a 38-year-old woman who presented with a left upper abdominal mass. The mass was initially suspected to be a malignant neoplasm because no history of either operation or trauma existed for this patient. Moreover, the location of the mass was unusual and the enhancement patterns of the dynamic magnetic resonance images were similar to that of a malignant tumor. Based on the radiologic findings, a surgical exploration was performed. A well-circumscribed mass, which measured 3.2 x 2 cm, was found in the rectus muscle. Microscopic findings showed a typical zonal pattern with a fibroblastic central zone and a zone of ossification at the periphery. Although abdominal myositis ossificans is extremely rare, it is one of the causes of abdominal mass lesions and can be mistaken for a malignant tumor. Therefore, a thorough knowledge of the evolution of myositis ossificans is necessary and in cases with malignant suggestion on magnetic resonance imaging, like that seen in our case, we suggest that a surgical excision may be necessary for both the diagnosis and treatment.