We present a case of a healthy 43-year-old man who experienced right lower abdominal mass with gastrointestinal upset for 6 months. A series of imaging studies revealed a large lobulated cyst in the right pelvis and retroperitoneum. Because of the persistent symptom, surgical intervention was performed to remove the cystic lesion. The final pathology report demonstrated a large seminal vesicle cyst with agenesis of kidney. It is compatible with the diagnosis of Zinner's syndrome. However, right lower abdominal mass is a rare manifestation of this syndrome. This case reminds us a unique differential diagnosis of a right lower abdominal mass.