Objectives: Point-of-care ultrasound (POCUS) diagnosis of distal forearm fractures relies on the identification of buckling or breach of hyperechoic bone cortex. We describe the pronator quadratus hematoma (PQH) formation visualized on POCUS, the PQH sign, as it may aid diagnosis of pediatric distal forearm cortical breach fractures. Methods: A prospective cohort of children presenting to an emergency department with isolated, clinically non-angulated distal forearm injuries received POCUS by an expert emergency physician sonologist who identified the presence or absence of the PQH sign. They secondarily recorded the difference between the size of the pronator quadratus (PQ) muscle on both the affected and non-affected forearms (PQ delta thickness). Children received an x-ray subsequent to POCUS and were diagnosed based on an x-ray reported by a radiologist masked to POCUS findings. Results: Thirty-eight children were recruited. All 22 patients with cortical breach fracture had PQH sign present (100%; 95%CI: 85–100%), while all 16 patients without cortical breach fracture had PQH sign absent (100%; 95%CI: 79–100%). PQ delta thickness ranged from 2.1 to 10.2 mm in cortical breach fractures, 0.0 to 1.1 mm in buckle fractures, and 0.2 to 0.8 mm in patients without fracture. Conclusions: The PQH sign correctly distinguished all children with, and without, cortical breach fractures. All PQ delta thicknesses were ≧2.1 mm when cortical breach fracture was present and ≦1.1 mm when cortical breach fracture was absent. The PQH sign and PQ delta thickness are promising measurements to identify pediatric distal forearm cortical breach fractures, and their utility should be confirmed in larger studies with sonologists of different abilities.