Mesenchymal chondrosarcoma is a rare malignant cartilaginous tumor arising within the bone or soft tissue. An 18-year-old woman presented with a tumor on her left fourth rib. We performed a wide resection of the tumor and administered three cycles of postoperative adjuvant chemotherapy. Three years after the operation, the patient is alive without any evidence of either local recurrence or distant metastases. The findings of this case may thus support the usefulness of a radical resection and adjuvant chemotherapy for mesenchymal chondrosarcoma.