The incidence of secondary neoplasms of the testis during autopsies is approximately 2.5%. Although most secondary testicular metastases are due to prostate cancer, only a few patients with prostate cancer have clinically manifested testicular metastasis. We report the case of a prostate cancer patient with testicular metastasis who was diagnosed after the presence of a palpable mass in the right testis. A 56-year-old Japanese male presented to our hospital with an elevated serum prostate-specific antigen (PSA) level of 137 ng/ml. He was diagnosed with stage IV (T3N1M1b) prostate cancer and received androgen deprivation therapy, followed by various hormonal manipulations. His serum PSA level was undetectable for 1 year. No distant metastases were detected during imaging examinations. He received radiation therapy; however, his serum PSA level increased gradually. Four months later, he presented with right testicular swelling. Computed tomography revealed a heterogenous mass in the right testis and a right high inguinal orchiectomy was performed. Histopathological analysis showed that the right testis was infiltrated with metastatic adenocarcinoma with a Gleason score of 8. This is a rare case of right testicular metastasis in a patient with prostate cancer. Testicular metastasis of prostate cancer can be aggressive and metastasize.