Primary small cell carcinomas are rare in domestic animals. A mass measuring 15 x 20 x 9 cm was detected in the left abdominal cavity of a 7.5-year-old female golden retriever. The cut surface of the excised mass showed a tumour replacing the left kidney. Microscopically, the mass was composed of polymorphic, small basophilic cells with a high nuclear to cytoplasmic ratio and round, oval or short slender fusiform nuclei with condensed or finely granular chromatin, absent or inconspicuous nucleoli, and scant, faintly eosinophilic cytoplasm with poorly defined cytoplasmic borders. Immunohistochemically, most of the neoplastic cells were immunoreactive for thyroid transcription factor 1 and CD56, moderately positive for vimentin and weakly or sparsely labelled for chromogranin A, synaptophysin, Wilms' tumour 1 protein, neuron-specific enolase, pan-cytokeratin (CK) AE1/AE3 and epithelial membrane antigen. The tumour cells were negative for glial fibrillary acidic protein (GFAP), ionized calcium-binding adapter molecule 1, CK7, CK20, CD3, CD45 and CD99. These findings indicated a neuroendocrine origin of the tumour. To the best of author's knowledge, this is the first report of a small cell neuroendocrine carcinoma originating as a primary tumour in the kidney of a dog. (C) 2020 Elsevier Ltd. All rights reserved.