Benign chest wall mesenchymoma in children is an extremely rare disease. Only 20 patients have been reported in the world literature. We report a chest wall mesenchymoma in a 2-year-old boy who was admitted to the hospital after a routine chest roentgenogram showed a mass in the right upper chest wall. The patient was asymptomatic. Clinical examination was negative, but chest roentgenograms and computed tomography showed a mass in the right upper chest wall involving the third rib. A 2 x 2 x 1.5-cm tumor was excised totally with partial resection of the third rib. The histology of the lesion corresponded to a mesenchymoma (hamartoma) of the chest wall. Our patient has been followed up for 8 years without recurrence.