The Klippel-Trenaunay syndrome is a combination of venous and capillary malformations associated with soft tissue and/or bony limb hypertrophy, with or without lymphatic malformations. Although persistent foetal veins are rare, the persistence of the lateral marginal vein is a common association in this syndrome. It results in venous hypertension, which gives rise to venous varicosities, which are commonly seen in this syndrome. This is a case report of a 28-year-old man with Klippel-Trenaunay syndrome, with persistence of the lateral marginal vein, affecting his right lower limb. He was treated with an above-knee amputation. The amputated limb was dissected to demonstrate the anatomy of the lateral marginal vein. To the best of the authors' knowledge, the gross anatomy of the lateral marginal vein has not been previously reported.