Leiomyosarcoma is an uncommon soft tissue sarcoma of mesenchymal cell origin, which shows smooth muscle differentiation. Leiomyosarcoma is seldom found in the pediatric population, and accounts for fewer than 2% of all soft tissue sarcomas. Leiomyosarcoma of the chest wall is extremely rare in children. We report here a case of an 8-year-old boy with a primary leiomyosarcoma that was incidentally found as a rib mass. The patient underwent a complete resection for a suspected osteochondroma diagnosed by a three-dimensional chest computed tomography examination. Pathological findings of the mass revealed intersecting fascicles of spindle cells showing cigar-shaped nuclei, inconspicuous nuclear pleomorphism and occasional mitotic figures in the background of a suspected osteochondroma of the rib. This report documents the first description of a leiomyosarcoma possibly arising in an osteochondroma of the rib in a child.