A 48-year-old male who presented with an enlarged right scrotum was diagnosed with malignant transformation of testicular teratoma. Physical examination revealed a right scrotal mass of hard consistency with no inguinal lymphadenopathy. Since prepuberty, his right testis had been larger than the left one, with no pain or tenderness. Computed tomography and bone scan revealed retroperitoneal lymphadenopathy and multiple bone metastases. Right orchiectomy was performed immediately, and a pathological examination revealed a mature teratoma associated with adenocarcinoma, showing signet ring cell differentiation. Cisplatin-based combination chemotherapy was administered; however, the metastatic lesions progressed, and the patient succumbed to the disease after 15 months. Only a few cases of primary malignant transformation of teratoma in the testis have been reported, and this is the first case report of primary malignant transformation of teratoma in the testis with signet ring cell-type differentiation.