Hyponatremia is one of the common electrolyte imbalances among hospitalized patients with the syndrome of inappropriate secretion of antidiuretic hormone (SIADH) being a common etiology for hyponatremia. There are multiple pathophysiologic considerations in the differential diagnosis of the etiologic factor for SIADH, including infections such as pneumonia and meningitis, as well as coronavirus disease 2019 (COVID-19) infection. However, SIADH, as the sole initial presentation of the infection of COVID-19, is rarely reported. In this report, we present a case of SIADH as the initial and only presentation of a COVID-19 infection, highlighting the clinical course and treatment strategy while providing the putative pathophysiologic insights into this unusual and potentially serious complication of COVID-19 infection.