Thromboembolism presenting with malignancy is common in adults but rare in children. We describe the case of a 17-year-old boy admitted to our hospital with syncope. Computed tomography revealed thromboembolism in both the lungs. Magnetic resonance imaging found thromboembolism in the inferior vena cava and a large heterogeneous mass in the pelvis. Pelvic osteosarcoma was confirmed by computed tomography-guided biopsy. Despite intensive chemotherapy and local radiation, only transient response was noted, the tumor remaining unresectable. To our knowledge, this is the first reported case of simultaneous pulmonary and inferior vena cava thromboembolism secondary to pelvic osteosarcoma in children. We also emphasize syncope as a unique feature of pulmonary thromboembolism. Accordingly, thromboembolism should be kept in mind as the first manifestation of occult malignancy, even in children.