A 9-year-old female neutered collie-cross dog was presented with a 2-month history of persistent diarrhoea, weight loss and intermittent vomiting. Abdominal ultrasonography revealed one loop of jejunum with a markedly thickened and multifocally hyperechoic wall, without loss of wall layering. Laparotomies were performed for biopsy and resection of affected intestine. Histopathological examination revealed small intestinal ganglioneuromatosis (GN). The dog recovered well from surgery and the diarrhoea resolved. Eleven months later the dog has gained weight and remains asymptomatic. This is the first report of small intestinal GN affecting a mature dog, in which pathology was localized to the mucosal lamina propria and surgical treatment resulted in a successful outcome.