We report an external iliac venous aneurysm in a young pregnant woman who was diagnosed incidentally by ultrasound scanning. The aneurysm was successfully treated by tangential aneurysmectomy and lateral venorrhaphy. Primary iliac venous aneurysm is a rare vascular abnormality. The clinical significance of the disease is unknown. However, embolism, rupture, and thrombosis might occur as they can occur with popliteal venous aneurysm. In fact, three of four reported patients with iliac venous aneurysms had a thromboembolic event. For those reasons, prophylactic treatment is indicated. This is the first patient with an iliac venous aneurysm to be diagnosed without complication.