Achondroplasia has a known association with foramen magnum stenosis that can result in cervicomedullary compression, which is most often due to a hypertrophied posterior occipital rim and an undersized transverse diameter. The authors present a unique case of a child with achondroplasia with symptomatic craniocervical compression from marked overgrowth of his opisthion anterior to the posterior arch of the atlas. This 22-month-old child with achondroplasia presented with severe respiratory and motor disabilities, including progressive quadriparesis and apneic episodes requiring continuous positive airway pressure. Magnetic resonance imaging and CT scans revealed marked foramen magnum stenosis from overgrowth of the opisthion, a hypoplastic C-1 ring, and spinal cord edema at the cervicomedullary junction. Foramen magnum decompression and a C-1 laminectomy were performed. Postoperatively, steady motor improvement has been observed and the patient no longer requires ventilatory support. To the authors' knowledge, this is the first report of this unusual anatomical entity.