Primary adenocarcinoma of the rete testis is a rare malignant testicular tumor with a poor prognosis. Here we report the case of a 54-year-old man with right hydronephrosis that was detected before the diagnosis of the primary testicular lesion. During the evaluation of the right hydronephrosis by use of abdominopelvic computed tomography, a painless, hard, solid lesion was found on the right testis. The patient underwent radical orchiectomy, and the pathologic examination revealed an adenocarcinoma of the rete testis. Multiple metastases were present at the time of diagnosis. The patient received combined chemotherapy after the surgery but lived only 8 months after the initial diagnosis.