We report a case of en bloc avulsion fracture of tibial tuberosity and Gerdy's tubercle, which has never been reported. A 14-year-old boy suffered from an acute pain in his left knee during running just before a jump. Simple radiographs showed an avulsion of the tibial tuberosity. On CT scans, the fractured fragment was attached not only to patellar tendon but also to iliotibial band (ITB) via Gerdy's tubercle. MRI evaluation revealed no intra-articular associated pathology. Open reduction and internal fixation with three cannulated screws were performed under lateral parapatellar approach to expose both the tibial tuberosity with patellar tendon and Gerdy's tubercle with ITB. At postoperative 1 year, he could walk, run, squat, and complained of no difficulty in activities on daily living with full range-of-motion of the knee. Radiographs showed well-healed fracture in situ. Gradually, he returned to sports activity. We believe that the injury was caused by the dynamic pull of quadriceps muscle via patellar tendon onto tibial tuberosity and the mostly static pull of ITB onto Gerdy's tubercle, both of which took a part in the fracture of the anterolateral portion of the unfused epiphysis of proximal tibia. The pes anserinus attaching on the anteromedial metaphysis of proximal tibia might exert the opposing deforming force. Preoperative planning including the determination of the extent of fracture and recognition of concomitant injury is a prerequisite for appropriate treatment.