Low-grade myofibroblastic sarcoma (LGMFS) is a fusiform cell tumor which develops in bone or soft tissues. This type of tumor frequently occurs in the oral cavity and extremities, while it is extremely rarely found in the abdominal cavity. This article reports a case of LGMFS exceeding 20 cm in diameter in the abdominal cavity observed in a 65-year-old male patient. The patient visited our hospital complaining of a heavy feeling of the stomach and abdominal distension. Imaging examinations revealed a giant solid tumor in the abdomen, and surgical treatment was scheduled. During the operation, a tumor about 20 cm in diameter with its anterior aspect covered with the greater omentum was found. The tumor had firm adhesions to the surrounding tissues, and it was excised with concomitant resections of the tail of the pancreas and the spleen. Histopathologically, fusiform cells were arranged in a complicated or storiform pattern, and immunohistochemical staining revealed that the tumor was positive for α-smooth muscle actin, negative for S100β, H-caldesmon and c-KIT, and a diagnosis of LGMFS was made.