BACKGROUND An ipsilateral fracture of the midshaft clavicle with dislocation of the acromioclavicular joint (ACJ) is a rare combination injury and almost always occurs following high-energy trauma. Currently, there is no optimal surgical approach for this kind of rare injury. We present a 60-year-old man with a traumatic combined linear midshaft clavicle fracture and ACJ injury simultaneously treated with Knowles pin fixation. CASE REPORT A 60-year-old male patient presented a linear midshaft clavicle fracture in the emergency room (ER) due to a road traffic accident. A linear fracture progressed to a displaced fracture at followup in the outpatient orthopedic department 3 days later. After open reduction with Knowles pin fixation for displaced clavicle fracture, postoperative followup radiographs revealed unexpected ipsilateral type V ACJ dislocation according to Rockwood classification. The next day, a closed reduction with percutaneous Knowles pin fixation was performed for ACJ dislocation. At the 1-year followup, radiographic and clinical results demonstrated complete union of the clavicle fracture and anatomic reduction of the ACJ with painless and full range of motion. CONCLUSIONS This report highlights that even a linear midshaft clavicle fracture can be combined with ipsilateral ACJ dislocation if the traumatic injury was caused by a high-energy road traffic accident. Therefore, an intraoperative stress view of the injured shoulder is recommended to recheck the stability of the ACJ after the clavicle fracture fixation to prevent a missed ACJ injury. In our case, an excellent outcome was achieved by using Knowles pin fixation simultaneously to treat the dual shoulder injury.