This review examines the prevalence, aetiology, pathophysiology, prognostic value, and investigation of dysnatraemia in hospitalised COVID-19 patients, taking into account all relevant studies published in PubMed and Cochrane Library studies until March 2021. Hyponatraemia is commonly observed in patients with bacterial pneumonia and is an independent predictor for excess mortality and morbidity. However, it remains unknown whether this association applies to coronavirus disease-2019 (COVID-19). Several studies reported a 20-35% prevalence for hyponatraemia and 2-5% for hypernatraemia in patients admitted with COVID-19. In addition, hyponatraemia on admission was a risk factor for progression to severe disease, being associated with an increased likelihood for the need for invasive mechanical ventilation, with an odds ratio (OR) of 1.83-3.30. Hyponatraemia seems to be an independent risk factor for mortality, with an OR of 1.40-1.50 compared to normonatraemia, while hypernatraemia is related to even worse outcomes than hyponatraemia. Furthermore, preliminary data show an inverse association between serum sodium and interleukin-6 levels, suggesting that hyponatraemia might be used as a surrogate marker for the risk of a cytokine storm and the need for treatment with interleukin antagonists. In conclusion, dysnatraemia is common and carries a poor prognosis in COVID-19 patients, indicating that it may play a future role in risk stratification and individualising therapy.