A 47-year-old man presented with dysuria. The prostate-specific antigen level was 65.5 ng/mL. Retropubic radical prostatectomy and regional lymphadenectomy revealed moderately differentiated adenocarcinoma (Gleason score 3 + 4 = 7, pT2N0). Postoperative adjuvant hormonal therapy was started immediately. Three years later, hormonal therapy was changed to anti-androgen monotherapy. Monotherapy was continued for eight years and then discontinued because the PSA level was maintained at <0.04 ng/mL. However, biochemical recurrence occurred 12 months after adjuvant hormonal therapy was discontinued. A computed tomography scan showed left inguinal lymphadenopathy. Biopsy of the swollen inguinal lymph node revealed metastatic prostate cancer. We report a rare case of metastatic prostate adenocarcinoma only to the inguinal lymph nodes.