Retroperitoneal lipomas are rare tumors. The etiology of their proliferation is uncertain. Metabolic and genetic causes have been reported. We present the case of a 12-year-old patient with a large pelvic lipoma causing dysuria and pelvic pain. Magnetic resonance imaging revealed the existence of a large retroperitoneal fat tumor. Ultrasound-guided fine needle biopsy was performed to exclude malignancy. The biopsy specimen revealed tissue consistent with a benign lipoma tumor. Exploratory laparotomy revealed a solid lipoma of the retroperitoneal space 8 × 6 cm and weighing 300 g. The patient had a good postoperative performance status. Lipomas should be considered in the differential diagnosis of a solid tumor in the retroperitoneal space.