Coronavirus disease 2019 (COVID-19) is a widespread disease. Hyponatremia in the setting of the syndrome of inappropriate antidiuretic hormone secretion (SIADH) was described with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Nonetheless, seizure as a prominent manifestation of hyponatremia associated with COVID-19 is rare. We present a case of a middle-aged man with mild COVID-19 pneumonia, who developed a seizure due to SIADH-related severe hyponatremia.