Objective The pandemic of coronavirus disease (COVID-19) has rapidly spread globally and infected millions of people. The prevalence and prognostic impact of dysnatremia in COVID-19 is inconclusive. Therefore, we investigated the prevalence and outcome of dysnatremia in COVID-19. Design The prospective, observational, cohort study included consecutive patients with clinical suspicion of COVID-19 triaged to a Swiss Emergency Department between March and July 2020. Methods Collected data included clinical, laboratory and disease severity scoring parameters on admission. COVID-19 cases were identified based on a positive nasopharyngeal swab test for SARS-CoV-2, patients with a negative swab test served as controls. The primary analysis was to assess the prognostic impact of dysnatremia on 30-day mortality using a cox proportional hazard model. Results 172 (17%) cases with COVID-19 and 849 (83%) controls were included. Patients with COVID-19 showed a higher prevalence of hyponatremia compared to controls (28.1% vs 17.5%, P < 0.001); while comparable for hypernatremia (2.9% vs 2.1%, P = 0.34). In COVID-19 but not in controls, hyponatremia was associated with a higher 30-day mortality (HR: 1.4, 95% CI: 1.10–16.62, P = 0.05). In both groups, hypernatremia on admission was associated with higher 30-day mortality (COVID-19 - HR: 11.5, 95% CI: 5.00–26.43, P < 0.001; controls - HR: 5.3, 95% CI: 1.60–17.64, P = 0.006). In both groups, hyponatremia and hypernatremia were significantly associated with adverse outcome, for example, intensive care unit admission, longer hospitalization and mechanical ventilation. Conclusion Our results underline the importance of dysnatremia as predictive marker in COVID-19. Treating physicians should be aware of appropriate treatment measures to be taken for patients with COVID-19 and dysnatremia.