Background: Malnutrition exacerbates the prognosis of various diseases; however, its specific impact on severe coronavirus disease (COVID-19) outcomes remains underexplored.Methods: This multicenter study in Korea assessed the nutritional status of 1,088 adults with severe COVID-19 using the Geriatric Nutritional Risk Index (GNRI) based on serum albumin levels and body weight.The patients were divided into the GNRI>98 (no-risk) and GNRI≤98 (risk) groups.Propensity score matching, adjusted for demographic and clinical variables, was performed. Results:Of the 1,088 patients, 642 (59%) were at risk of malnutrition.Propensity score matching revealed significant differences in hospital (34.3% vs. 19.4%,p < 0.001) and intensive care unit (ICU) (31.5% vs. 18.9 %, p < 0.001) mortality between the two groups.Risk group was associated with increased hospital mortality rate in the multivariate Cox regression analyses after propensity matching (hazard ratio (HR): 1.64, p = 0.001).Among the 670 elderly patients, 450 were at risk of malnutrition.Moreover, the risk group exhibited a higher hospital mortality (52.1% vs. 29.5%,p < 0.001) and ICU mortality rates (47.2% vs. 29.1%,p < 0.001).Risk group was significantly associated with increased hospital mortality rates in the multivariate analyses after propensity matching (HR: 1.66, p = 0.001). Conclusion: Malnutrition, as indicated by a low GNRI, was associated with increased mortality in patients with severe COVID-19.A similar effect was also observed in the 4 elderly population.These findings highlight the importance of nutritional assessment and effective interventions for patients with severe COVID-19.