Myositis ossificans is a pseudoinflammatory tumour that originates from skeletal muscle and corresponds to a heterotopic, metaplastic, nonmalignant bone tumour. The purpose of this article is to report the case of myositis ossificans circumscripta (MOC) of the psoas muscle due to overuse in an adolescent gymnast. A 16-year-old female athlete presented at our outpatient orthopaedic clinic for evaluation of a 1-month history of low back pain. Initial plain radiographs were initially interpreted as negative, and laboratory values were normal. MRI imaging demonstrated a circumscribed mass with associated oedema in the psoas muscle. Computed tomography-guided percutaneous biopsy was performed and histology confirmed the diagnosis of MOC. Conservative treatment was initiated with rest and anti-inflammatory drugs (indomethacin). The patient had a resolution of pain and function after 3 months of conservative treatment. At 6 months’ follow-up, MRI demonstrated complete resolution of the lesion and she gradually returned to her sports activity. At last follow-up she was asymptomatic. MOC is a rare lesion in the paediatric-adolescent population. To our knowledge, this is the first report of MOC in the psoas muscle produced by overuse. MRI is very sensitive in detecting oedema during the acute phase of the lesion. Conservative treatment should be considered, especially at the early stage of the disease. Spontaneous resolution can be expected in most cases. Level of Evidence: IV.