Discoveries of juvenile primary bone malignant neoplasm have been rare in archaeological settings. This study presents the first case as such from ancient China. An individual (M799) aged 14–17 years from the Dongxin cemetery of the Northern Wei Dynasty (386–534 CE), Datong, Shanxi Province, was found with signs of the bone malignant neoplasm. The skeleton of the juvenile was macroscopically examined with the aid of computed tomography imaging. Results demonstrated that a bony outgrowth was found on the distal one third of the diaphysis of the left femur; both osteoblastic and osteolytic processes were present on the lesion (about 148 mm long), and the highly dense and lobulated exophytic mass protruded about 50 mm from the posterior aspect of the diaphysis. A periosteal new bone reaction was found on the adjacent cortex while the medullary cavity was invaded by the lesion. Osteosarcoma was considered the most likely diagnosis, with differential diagnoses of Ewing sarcoma and chondrosarcoma. Otherwise, the disease had no asymmetric effect on femoral length. This rare case enriches our knowledge on the paleoepidemiological patterns of osteosarcoma and other malignant neoplasms from the past.