Intravenous leiomyomatosis is a benign smooth muscle tumor that often occurs in the internal iliac vein and is closely associated with a fibroid. Intravenous leiomyomatosis usually starts in the veins of the uterus. It can grow within the veins and extend into the inferior vena cava and ultimately extend into the right-sided heart chambers and pulmonary arteries. Other sites are less common, and a venous primary site is very rare. We report a case of subclavian vein tumor that was diagnosed as an unusual leiomyoma in a 21-year-old nulliparous woman.