Myositis ossificans circumscripta (MOC), characterised by non-neoplastic heterotopic bone formation in soft tissue and skeletal muscle, is rare in children. At onset, it is difficult to distinguish MOC from a musculoskeletal infection or neoplasm, particularly in absence of trauma, and a biopsy is frequently required. We describe an 11-year-old boy with MOC in his thigh. At admission, minor local trauma was referred, and clinical examination revealed a tender and painful mass in the left thigh. Hypothesising a periarticular infection, a wide-spectrum antibiotic and an anti-inflammatory drug were given. The magnetic resonance imaging revealed an enlarged mass with marked enhancement of the lesion and a central, irregular non-enhanced area due to necrosis. After incisional biopsy, the histopathological examination found immature osteoblasts inside striated muscle fibres, as well as proliferating fibroblasts, which are all compatible with MOC, thereby ruling out infection or malignancy. The CT scan showed calcified deposits arranged in a ring, thus confirming MOC; a radiological follow-up was suggested to detect the lesion's maturation. Three months after diagnosis, surgical excision was performed. The post-operative period was uneventful. Six months later, the child was in good condition, with no signs of recurrence. In the literature, 57 paediatric cases have been described; most of these had an acute course, required excision and then had a favourable evolution. Our case study confirms the good prognosis of MOC and underlines how this benign condition should be considered in children presenting a tender and painful soft-tissue swelling.