We report an unusual case of a pelvic mass arising from the seminal vesicle. A 45-year-old man presented with difficulty voiding and difficult bowel movements. Computed tomography demonstrated a large 14 × 10 × 15-cm cystic pelvic mass displacing the bladder anteriorly and the rectum posteriorly. Magnetic resonance imaging confirmed that this lesion appeared to be originating from the right seminal vesicle. On surgical exploration of the abdomen, a large cystic lesion in the posterior retroperitoneum was identified and removed. The patient improved, with restoration of normal bowel function.