Ewing sarcoma is a malignant round-cell tumor. It is a rare disease in which cancer cells are found in the bone or in soft tissue. The most common areas in which it occurs are the pelvis, the femur, the humerus, and the ribs. Because a common genetic locus is responsible for a large percentage of Ewing sarcoma and primitive neuroectodermal tumors, these are sometimes grouped together in a category known as the Ewing family of tumors.

The diseases are, however, considered to be different: peripheral primitive neuroectodermal tumors are generally not associated with bones, while Ewing sarcomas are most commonly related to bone. Ewing sarcoma occurs most frequently in male teenagers, with a male/female ratio of 1.6:1.

Although usually classified as a bone tumor, Ewing sarcoma can have characteristics of both mesodermal and ectodermal origin, making it difficult to classify.