We report a rare case of syndrome of inappropriate antidiuretic hormone secretion (SIADH) associated with amyotrophic lateral sclerosis (ALS). A 69-year-old man was admitted to our hospital with sustained hyponatremia. Hyposmolality with elevated urinary osmolality and sodium excretion was observed, which indicated SIADH. The treatment for SIADH was challenging; the patient developed carbon dioxide narcosis, which led to the diagnosis of ALS. After the initiation of noninvasive positive-pressure ventilation, the patient's serum sodium concentration normalized and became stable. Thus, ALS should be recognized as a possible cause of SIADH in the clinical setting.