Here, we report an adult case of intra-abdominal testicular cancer after surgical treatment of an undescended testis in infancy. A 36-year-old male patient was accidentally diagnosed with a tumor in the pelvic cavity by ultrasonographic examination. He had hematuria and the human chorionic gonadotropin beta subunit level was slightly elevated. T2-weighedmagnetic resonance imaging revealed a well-defined and highly intense mass. Since these findings suggested intra-abdominal testicular cancer, laparoscopic surgery was performed to remove the mass. Laparoscopy revealed an intra-abdominal tumor accompanied by a looping vas deferens entering the left inguinal canal. The distal part of the looping vas had already been removed from the external inguinal ring. The pathological findings revealed a pT1 seminoma. The patient has been recurrence-free for 12 months. The present case implies the importance of careful investigation and treatment for intra-abdominal testicular cancer, since intra-abdominal testis might have been overlooked at the time of surgery for undescended testis.