The accurate diagnosis and treatment of posterior plagiocephaly have been a source of controversy. The unilateral lambdoid synostosis that is characterized by flattening of ipsilateral occipital bone is a rare type of craniosynostosis. An 8-month-old infant with unilateral lambdoid synostosis is reported. The patient exhibited right occipital flattening, mastoid bulge, and right frontal bone prominence, with the right auricle displaced anteroinferiorly on gross examination. A Towne projection radiograph of the skull and three-dimensional computed tomography scans revealed that the right lambdoid suture was prematurely fused. Correction of the cranial shape was performed using distraction osteogenesis combined with a barrel stave osteotomy. Significant improvement in the skull was observed 1 year after surgery.