Retroperitoneal vascular leiomyosarcoma (RVLMS) are rare soft-tissue sarcomas that most commonly arise from large blood vessels and have a poor prognosis. We present the case of a 61-year-old woman who presented with isolated left flank pain. Abdominal computed tomography and magnetic resonance imaging revealed a 5 cm retroperitoneal soft-tissue mass that was contiguous with the left ovarian vein and connected to the proximal ureter, causing hydronephrosis. As ureteroscopy suggested extrinsic compression of the ureter, a percutaneous biopsy of the mass was obtained, whereupon diagnosis of leiomyosarcoma was made. Radical en-bloc excision of the tumor, including the involved upper urinary tract and the gonadal vein, was performed. The tumor proved to be a leiomyosarcoma arising from the ovarian vein wall. No adjuvant therapy was planned, and no recurrence was noticed at her 24-month follow-up. Primary RVLMS of the ovarian vein is an uncommon condition. To date, only a few sporadic cases have been reported in the literature. What makes the present case further interesting is the unusual tumor’s relationship with the patient’s ureter, raising both diagnostic and management challenges. To our best knowledge, this is so far the fourth reported case of its kind to cause ureteral obstruction.