A 13-month-old, male bichon frise was examined for the investigation of intermittent seizures, ataxia, abnormal behavior, polyuria, and polydipsia. At presentation, clinical and neurological examinations were unremarkable with the exception of mild truncal ataxia and a domed skull. Severe hyponatremia and hypoosmolality were identified, and following diagnostic testing a diagnosis of the syndrome of inappropriate antidiuretic hormone secretion (SIADH) was made. Magnetic resonance imaging revealed changes consistent with severe hydrocephalus. Water restriction resulted in increased serum osmolality and a reduction in severity of clinical signs. The current case report documents SIADH associated with hydrocephalus in a dog. Structural brain disease should be excluded before a diagnosis of idiopathic SIADH is made.