Purpose. We present a case with Klippel-Trenaunay (KT) syndrome that had unilateral mature cataract and vitreoretinopathy. Case Report. A 17-year-old boy with KT syndrome presented to the clinic of ophthalmology for low vision in the right eye. His best corrected visual acuity (BCVA) was hand motion in the right eye and 20/20 in the left eye. Anterior segment examination revealed mature cataract in the right. During the physical examination, port-wine stains were noted over right side of his face, ankle, and toes. He had asymmetric face and his head was larger on the right side. Leg lengths were symmetrical, although he had skin hypertrophy. Cranial magnetic resonance imaging studies showed cortical atrophy discordant to his age, asymmetric vascular dilatations in the right hemisphere, hypertrophy in the right periorbital soft tissue, and choroidal plexus. The patient received an uncomplicated cataract surgery. His BCVA in the right eye improved to 20/200 after the surgery. After removing cataractous lens, we were able to examine the fundus that revealed severe vitreoretinopathy and choroidal hemangioma. Conclusion. This case emphasizes the importance of prompt ophthalmic examination in patients with KT syndrome.