Generalized lymphadenopathy is a rare manifestation of metastatic prostate cancer. Here, we report the case of a 59-year-old male patient with supraclavicular, mediastinal, hilar, and retroperitoneal and inguinal lymphadenopathy, which suggested the diagnosis of lymphoma. There were no urinary symptoms. A biopsy of the inguinal lymph node was compatible with adenocarcinoma, whose prostatic origin was shown by immunohistochemical staining with PSA. The origin of the primary tumor was confirmed by directed prostate biopsy. We emphasize that a suspicion of prostate cancer in men with adenocarcinoma of undetermined origin is important for an adequate diagnostic and therapeutic approach.