A case of an extra-abdominal desmoid tumor presenting as an intrathoracic tumor (intrathoracic desmoid tumor) in a 46-year-old woman is reported. The tumor originated in the left chest wall and protruded into the left pleural cavity. Simple resection was carried out. The tumor, measuring 13 x 9 x 7 cm, was solid, gray-tan in color, and covered with parietal pleura. Histologically, the tumor was composed of a hypocellular arrangement of spindle-shaped cells with a fibromyxoid background. In some areas, keloid-like hyalinized collagen fibers proliferated, and a perivascular hypercellular area was seen. Immunohistochemical analysis showed that the cytoplasms of the tumor cells were strongly positive for vimentin, and some tumor cells were positive for alpha-smooth muscle actin, but all tumor cells were negative for CD34. These findings were consistent with the characteristics of an intrathoracic desmoid tumor. The differential diagnoses, in particular solitary fibrous tumor and tumors with a myofibroblastic nature, are discussed.