A 30 year-old man was admitted to our hospital with a chief complaint of left flank pain. Computed tomography revealed a retroperitoneal tumor. Levels of lactic dehydrogenase, human β-chorionic gonadotropin, α-fetoprotein, DUPAN-2 and carbohydrate antigen 19–9 were elevated. No abnormal findings were present according to palpation and ultrasonography of the testes. The patient was diagnosed as having a retroperitoneal extragonadal germ cell tumor (EGCT) considering the elevated markers. Resection of the tumor, two cycles of neoadjuvant and one cycle of adjuvant chemotherapy (cisplatin and etoposide) were performed. The surgical specimen showed total necrotic tissue. Eight years later, the patient noticed an enlargement of his left testicle. The tumor felt elastic and firm on his left testis. Neither distant metastasis nor lymph node metastasis was present according to computed tomography. Left high orchiectomy was performed and histology revealed seminoma. Twenty-three cases have been reported previously about metachronous testicular tumor developing after retroperitoneal EGCT. We report the 24th case.