Synostotic frontal plagiocephaly is most commonly caused by frontoparietal (unicoronal) synostosis, but may be caused by other fusions along the coronal hemiring. Frontosphenoidal synostosis is a rare cause of frontal plagiocephaly, with only five previously reported cases. We present the case of an 8-month-old male infant with frontal plagiocephaly caused by unilateral isolated frontosphenoidal synostosis mimicking frontoparietal (unicoronal) synostosis. The clinical and radiological features included flattened ipsilateral forehead, retruded ipsilateral maxilla, a midline chin, contralateral deviation of the nasal root and endocranial base, and anomalies of the bony orbit. Unilateral isolated frontosphenoidal synostosis may be misdiagnosed as unicoronal synostosis. Careful physical and radiological assessment is necessary to recognise this synostotic disorder of the coronal hemiring.