Salter type 3 fractures of the proximal humerus are rare injuries. We report the first case of a Salter type 3 physeal fracture with posterior dislocation of the proximal humerus in a 16-year-old boy. The fracture pattern could not be evaluated by simple radiography, but was possible with the aid of MRI. The dislocated humeral head was locked behind the glenoid labrum. Open reduction was mandatory. Sixteen months after the operation, avascular necrosis of the humeral head was detected by simple radiography and MRI. Even though the patient had a full range of motion of the shoulder and no pain, a long-term follow-up is believed necessary.