We report a case of dermoid cyst of the right testis. An 85-year-old man was admitted to our hospital with an enlarged right testicle and a high prostate specific antigen (PSA) level. A hard testis of approximately 10 cm, with a smooth surface, was palpated. On digital rectal examination, a hard prostate of approximately 4 cm, with irregular surface, was palpated. We performed right inguinal orchiectomy and transrectal ultrasound-guided prostate needle biopsy. The wall of the testis was calcified, with a cyst that contained yellow-white sebaceous material and tufts of hair. Microscopically, the cyst wall showed calcification and ossification, and had hair roots. The diagnosis was dermoid cyst. We also found a well differentiated adenocarcinoma (Gleason 2–3) in the prostate. Dermoid cyst of the testis is very rare. To our knowledge, this case is in the oldest patient thus far reported.