<jats:title>Abstract</jats:title><jats:sec><jats:title>Background</jats:title><jats:p>This study aimed to investigate the predictive validity of the previously reported body mass index (BMI) cutoff values of the Global Leadership Initiative on Malnutrition (GLIM) criteria for patients with pneumonia in Asians, using a nationwide registry database.</jats:p></jats:sec><jats:sec><jats:title>Methods</jats:title><jats:p>The study included 26,098 (2282 patients aged <70 years and 23,784 patients aged ≥70 years) patients with pneumonia aged ≥65 years who were enrolled in the Japan Medical Data Center database between April 2014 and December 2018. Malnutrition was diagnosed using the GLIM criteria, and the severity of malnutrition was diagnosed by using a BMI cutoff value of <17.0 and <17.8 for patients aged <70 and ≥70 years, respectively. Multivariate analysis was performed to determine whether the severity of malnutrition based on BMI was associated with poor clinical outcomes, such as 30‐day in‐hospital mortality, length of hospital stay (LOS), and 30‐day readmission.</jats:p></jats:sec><jats:sec><jats:title>Results</jats:title><jats:p>A total of 14.7% and 24.1% of patients aged <70 and ≥70 years, respectively, experienced severe malnutrition. Severe malnutrition was independently associated with 30‐day in‐hospital mortality (hazard ratio [HR], 1.19; 95% CI, 1.05–1.34), prolonged LOS (coefficient, 5.13; 95% CI, 4.31–5.94), and 30‐day readmission (HR, 2.28; 95% CI, 1.89–2.75) in patients aged ≥70 years; however, only prolonged LOS was independently associated with patients aged <70 years (coefficient, 3.27; 95% CI, −0.24–6.78).</jats:p></jats:sec><jats:sec><jats:title>Conclusion</jats:title><jats:p>The BMI cutoff values might be valid for patients with pneumonia aged ≥70 years in Asians. Further investigation is required to validate the cutoff value for older adults aged <70 years.</jats:p></jats:sec>