Background: Forearm nonunion is a rare condition in children, and its treatment is usually successful. However, nontraumatic nonunion can be resistant to treatment. Although the vascularized fibular bone graft has been frequently used for complex forearm nonunion, there is no consensus on the best treatment method. In this study, we aimed to investigate the outcome of the antegrade posterior interosseous bone flap (PIBF) in treating children with complex radius nonunion. Methods: The participants consisted of 3 girls and 1 boy ranging from 31 months to 9 years of age. They were treated with PIBF surgery, and the nonunion of the radius was fixed with a plate or an external fixator. The nonunion was due to congenital pseudoarthrosis, osteogenesis imperfecta, or infantile osteomyelitis. All patients were evaluated for a clinical and radiologic union. Results: The radiologic union was achieved 2 to 3 months after the surgery in all cases. The functional recovery of the elbow, forearm, and wrist was satisfactory except for 1 patient who experienced donor site fracture, and finally radioulnar synostosis accompanied by radial head dislocation. Conclusion: Because of the simplicity of its technique and short duration of surgery, PIBF can be a reliable and less-demanding alternative to vascularized fibular bone graft in the treatment of complex forearm nonunion in children. Level of Evidence: Level IV.