A 5 yr old male neutered Labradoodle presented for an episode of acute collapse. Point-of-care blood work showed hypoglycemia and abdominal ultrasonography revealed a liver mass arising from the caudate liver lobe. The dog underwent a partial liver lobectomy, and histopathology confirmed a fully resected hepatocellular carcinoma. Blood glucose levels normalized initially after surgery, but 1 wk later, the patient was diagnosed with diabetes mellitus based on the development of polyuria, polydipsia, hyperglycemia, and glucosuria. Appropriate treatment with insulin was initiated, and 1 yr following the diagnosis, the dog was still requiring administration of insulin twice daily. This case describes the uncommon development of diabetes mellitus in a dog following surgical resection of a hepatocellular carcinoma initially associated with hypoglycemia. Although very unusual, this should be considered as a potential complication of surgical treatment of such tumors, and affected patients may require long-term medical management.