We report four cases of extraskeletal osteosarcoma of the thigh to illustrate MRI findings of this rare neoplasm. Calcifications or osteoid material were not discernible in these tumors on MRI or conventional radiography. Three tumors were well demarcated on MRI and corresponded to pseudoencapsulation on radiologic-pathologic correlation. These three tumors were heterogeneous in appearance and were hyperintense to muscle on T1-weighted imaging and demonstrated high signal intensity on T2-weighted imaging. In the fourth tumor, which occupied almost the entire thigh, MRI before and after intravenous gadopentetate dimeglumine administration revealed cystic, hemorrhagic, and solid components. Pathological examination revealed a cystic hemorrhagic cavity containing necrotic and viable tumor and a large solid tumor component. The MRI findings in extraskeletal osteosarcoma are non-specific. However, in the appropriate age group the differential diagnosis of a soft-tissue mass in the thigh should include extraskeletal osteosarcoma, even in the absence of radiographically discernible calcifications or osteoid material within the soft-tissue mass.