Desmoplastic fibroma is a very rare primary tumor of bone, closely related to aggressive fibromatosis of soft tissue. Although considered a benign lesion, it can be very aggressive locally and has a high rate of local recurrence after incomplete surgical excision. Radiologically, the lesion shows no distinctive features, often simulating osteosarcoma, fibrous dysplasia, or fibrosarcoma. Rib involvement by desmoplastic fibroma is extremely rare. We present two cases of desmoplastic fibroma involving this unusual location with lytic costal lesion and chest wall extension. Histological examination after surgical resection revealed that the tumor consisted of spindle cells with small, elongated nuclei in a background of numerous collagen fibers and infiltrating lamellar bone. There was no evidence of nuclear atypia, mitotic activity, or necrosis. We also discuss histological differential diagnosis as well as clinical features and the radiological and pathologic findings of this rare disease. Recognition of this entity is important to ensure proper surgical treatment.