Sternoclavicular joint dislocation is a rare event. It occurs most often in a violent trauma. The authors report the case of a10 years old child, received at emergencies for right shoulder blunt trauma after been punched by another child. He presented with right shoulder pain, right upper limb functional impairment and right sternoclavicular joint depression. Standard chest radiographs were normal. Chest CT scan showed posterior dislocation and allowed us to determine its variety. Twelve hours after the trauma, a closed reduction has been done under general anesthesia. A control CT scan showed a restoration of normal joint anatomy. After 18 months, the shoulder was painless and mobile in all directions. It is an isolated recent posterior sternoclavicular joint dislocation in a child. With this observation the authors emphasizeon the unusual mechanism of such a dislocation occurrence, the primary role of CT scan in the diagnosis and early closed reduction.