Purpose: Through an anatomical review, the aim of this study is to define the ulnar periosteal branches of the posterior interosseous vessels (PIV). In addition, we report the clinical utility of a vascularized ulnar periosteal pedicled flap (VUPPF), supplied by the investigated PIV, in a complex case of radial nonunion. Methods: Ten upper limbs latex colored from fresh human cadavers were used. Branches of the PIV were dissected under 2.5× loupe magnification, noting the periosteal, muscular, and cutaneous branches arising distal to the interosseous recurrent artery. The VUPPF was measured in length (cm) and width (cm). Results: The PIV provided a mean 12.8 periosteal branches to the ulna distributed along the most distal 15 cm, with a mean distance between branches of 1 cm, allowing for the design of a VUPPF which measured a mean 12 cm in length and 1.7 cm in width. We used a VUPPF of 7.8 cm in length and 2 cm in width to treat extensive nonvascularized bone graft nonunion with a defect of 2 cm of the left radius in a 6-year-old girl, secondary to previous Ewing's Sarcoma reconstruction. Successfully consolidation was achieved 6-months after surgery. The patient did not present postoperative complications. At 2-years of follow-up after surgery, active supination was 80° and pronation 0° (due an incomplete interosseous ossification); grip strength was 80% that of the opposite hand. The patient had resumed all her daily activities. Conclusions: VUPPF may be considered a valuable and reliable surgical option for forearm reconstruction in complex clinical scenarios.