Pediatric subtrochanteric fractures are relatively rare. There are some surgical options with various plate techniques. Here, we report the first description of a pediatric pathological subtrochanteric fracture treated with an adult proximal humerus polyaxial locking plate and describe the good clinical outcomes achieved. A 10-year-old boy had a pathological subtrochanteric fracture. A non-contact bridging proximal humerus osteosynthesis plate was used. Although this is designed for the adult proximal humerus, its size and shape were considered to fit the pediatric proximal femur. In addition, this is a polyaxial locking plate with a choice of screw insertion directions. During surgery, it was possible to determine an appropriate plate installation position and screw direction in consideration of the location of pathological lesions, the bone shape, and the femoral neck angle. Twelve months postoperatively, the fracture was healed, and pathological lesion consolidated without obvious growth failure.