Myositis ossificans (MO) is a rare benign cause of heterotopic bone formation within soft tissue. It most commonly affects adolescents and young adults, typically in the limbs and following trauma. Very few cases have been reported in children. We report here a case of nontraumatic MO occurring in a 10-year-old girl with an uncommon location in the 5th right intercostal space; it was initially misdiagnosed and treated as osteosarcoma. Imaging findings including plain radiographs, CT, MRI, bone scintigraphy and PET-CT are described. This case highlights the central role played by imaging in diagnosis, thus avoiding biopsy that can erroneously suggest osteosarcoma as the diagnosis, as occurred in this case.