A 28-year-old woman with a history of prior bilateral retinoblastoma presented with general fatigue, anemia, and a urinary bladder mass seen on abdominal ultrasound and CT. She was referred for a staging 18F-FDG PET/CT, which showed an intensely FDG-avid bladder mass that was biopsied to reveal a leiomyosarcoma, with no evidence of metastases, which guided her management. 18F-FDG PET/CT is routinely used in the evaluation of leiomyosarcomas; however, its use in the staging of a leiomyosarcoma of the urinary bladder has not been previously described in the literature. This case highlights the usefulness of PET/CT in the staging of this rare tumor.