Neuroendocrine neoplasms are rare epithelial neoplasms with neuroendocrine differentiation. Few cases of primary testicular poorly differentiated neuroendocrine carcinomas (PD-NECs) have been reported, and secondary testicular neoplasms are rare. A 61-year-old man with a chief complaint of left testicular swelling was referred to our hospital. An orchiectomy was performed in order to determine the pathological diagnosis. Pathological examination showed diffuse sheets of highly atypical cells that were positive for neuroendocrine markers and a Ki-67 proliferation index of 80%. The patient was diagnosed with poorly differentiated small-cell NEC. Contrast-enhanced computed tomography revealed multiple metastases to the pancreas, adrenal glands, and lymph nodes. Esophagogastroduodenoscopy showed multiple gastric metastases, and biopsy revealed the same histological findings as observed for the testicular tumor. Contrast-enhanced magnetic resonance imaging of the head also revealed multiple brain metastases. The confirmed diagnosis was PD-NEC of unknown primary with metastases to the testis, stomach, pancreas, adrenal glands, brain, and lymph nodes. We started the first-line chemotherapy with etoposide and cisplatin. Stereotactic radiotherapy for the brain metastases was administered between the first and second cycles. After five cycles, a partial response was observed; however, disease progression was observed after seven cycles with recurrence of the brain metastases and enlargement of all tumors. To our knowledge, this is the first report of an unknown primary PD-NEC with metastasis to the testis.