Leiomyosarcoma arising from the external iliac vein is uncommon. This is a report of a 51-year-old Japanese man with venous leiomyosarcoma originating from a branch vessel of the left external iliac vein. The tumor was found during a medical examination, and the patient had no symptoms. Computed tomography showed a 72 × 49 mm mass adjacent to the left external iliac vein. The tumor was resected en-block along with ligation of the external iliac vein due to strong adhesion with the tumor. Histological examination showed venous leiomyosarcoma, and its origin was thought to be a branch vessel of the left external iliac vein. The patient has remained free from recurrence at 30 months after surgery.