The syndrome of inappropriate antidiuretic hormone secretion (SIADH) is one of the most important causes of hyponatremia. SIADH has been associated many clinical states, syndromes and many other central nervous disorders. Neuromyelitis optica (NMO) and neuromyelitis optica spectrum disorders (NMOSD) are also a few of these. NMO is a demyelinating autoimmune disorder affected the brainstem and hypothalamus where express high levels of aquaporin-4 (AQP-4) protein. Although detecting antiaquaporin-4 antibody positive is important for the diagnosis, some NMO and NMOSD patients show seronegative AQP-4 antibody results. We report a seronegative NMO spectrum disorder case presenting with the SIADH.