Judo is a combat sport with high risk of injury. We present a rare case of traumatic left posterior sternoclavicular (SC) joint dislocation, inflicted to a 12-year-old boy during a judo contest. An extensive literature review did not reveal any case of posterior SC joint dislocation in judo. The patient was treated with closed reduction under general anesthesia. At 2-year follow-up, his left upper extremity had full range of motion, and he did not complain of any residual symptoms. He decided to discontinue judo training; however, he participates in other physically demanding sports. Although not often encountered, posterior SC joint dislocation is a challenging and critical medical problem that can be fatal if not promptly diagnosed and treated on time and should be considered in the differential diagnosis of trauma-related anterior chest pain.