Background: The clinical significance of hyponatremia in hospitalized patients with coronavirus disease 2019 (COVID-19) is unclear. Objective: To clarify the frequency, mechanisms, and clinical implications of hyponatremia in patients admitted to the hospital with COVID-19. Methods: PUBMED search until January 15, 2021. Search terms include COVID-19, hyponatremia, interleukin-6, anti-diuretic hormone, severity, mortality, retrospective, prospective studies, and meta-analyses are included. Results: Several large retrospective studies and one prospective investigation indicate that prevalence of hyponatremia in patients with COVID-19 ranges from 9.9% to 35.8%. In most studies, hyponatremia represents the most common electrolyte abnormality in patients admitted with COVID-19 and is mostly mild in severity biochemically. Hyponatremia is associated with severe picture of COVID-19 as reflected by increased need for intensive care and mechanical ventilation. Hyponatremia on hospital admission is an independent factor implicated in increased mortality of patients with COVID-19. Mechanisms of hyponatremia in COVID-19 may include inappropriate anti-diuretic hormone activity, increased interleukin-6 (IL-6), and less frequently concomitant adrenal insufficiency. Conclusions: Hyponatremia is a common electrolyte abnormality in patients with COVID-19 that is linked to poor prognosis. Further studies are needed to elucidate whether hyponatremia plays a causative role in worsening outcomes of COVID-19 or it is a marker of bad prognosis.