Abstract Introduction: Scrotal smooth muscle tumours are uncommon in man. They arise from the tunica dartos of the scrotum. They have not been reported in animals. Materials and Methods: A 6-year-old male shar pei dog presented with a single 1-cm diameter mass arising from the left scrotum. Surgical excision was performed and the nodule was fixed in 10% neutral buffered formalin, processed routinely and embedded in paraffin wax. Sections were stained with haematoxylin and eosin. Several sections were subjected to immunohistochemistry. Results: The neoplasm was well circumscribed, but unencapsulated. It showed multifocal contiguity with the dartos smooth muscle bundles. It comprised of short interlacing bundles of plump spindle cells within minimal collagenous stroma. The neoplastic cells had abundant eosinophilic cytoplasm and oval to elongated nuclei. Moderate anisokaryosis and anisocytosis were seen. There were seven mitotic figures per 10 high-power fields (?40). Small focal areas of necrosis were present. The neoplastic cells were positive for vimentin and smooth muscle actin, while they were negative for melan A, glial fibrillary acidic protein (GFAP) and S100. A few cells were positive for desmin. Conclusions: The histological and immunohistochemical findings were most consistent with a scrotal leiomyosarcoma. This is the first report of scrotal leiomyosarcoma in a dog.