The authors describe a case of synchronously occurring (double) tumours, i.e. primary hepatocellular carcinoma and aortic body chemodectoma in a 14-year-old mixed-breed male dog. The tumours were identified during necropsy, following euthanasia. In the last months of its life, the dog showed signs of weakness, anorexia, apathy, inactivity, and abdominal palpation elicited a painful reaction. The primary liver cancer emerged in the left lateral lobe without evidence of any distant metastases. Histopathological and immunohistochemical investigations revealed a well-differentiated, trabecular, claudin-7-, claudin-5- and pancytokeratin-negative hepatocellular carcinoma. The Ki-67 proliferation index was 33%. During necropsy, a synchronously occurring benign, grade I type aortic body chemodectoma was also detected in the dog. This neuroendocrine tumour showed chromogranin-, synaptophysin-, neuron-specific enolase- and S100 protein-positivity, and the Ki-67 proliferation index was 2%. The authors believe that this is the first description of synchronously occurring hepatocellular carcinoma and aortic body chemodectoma in a dog.