Background/Objectives: The clinical characteristics and prognostic factors of aspiration pneumonia remain poorly defined. Geriatric nutrition risk index (GNRI) has recently been reported to exhibit a prognostic value for several diseases in older adults. Thus, we aimed to investigate the clinical characteristics and prognostic significance of GNRI for aspiration pneumonia in a sample of older adult patients. Design: Retrospective observational cohort study. Setting: Single-institute acute-phase community hospital. Participants: Patients with aspiration pneumonia diagnosed at our institute between April 2014 and March 2016. Measurements: Data on patient characteristics, microbiological findings, and clinical course were extracted from electronic medical records. The primary outcome was in-hospital mortality. Receiver operating characteristic curve (ROC) analysis was conducted to compare the predictive value of each parameter. Logistic regression analysis was performed to identify independent prognostic factors. Results: Overall, 587 patients with aspiration pneumonia aged ≥ 65 years were enrolled. The mean age was 86 years. Among the patients, 97 patients (16.5%) died. ROC analysis for in-hospital mortality revealed that GNRI had a greater area under the curve value than albumin, body mass index, and A-DROP score, with a significant difference between GNRI and albumin (p=0.0058). Male gender sex (odds ratio [OR]: 1.88, p = 0.028,), chronic heart failure (OR: 2.14, p = 0.023), history of malignancy (OR: 2.66, p = 0.0025), lower GNRI (OR: 0.94, p < 0.001), and initial antibiotic change (OR: 4.22, p < 0.001) were identified as independent adverse prognostic factors in the multivariate analysis. Conclusion: Our findings indicate that GNRI is a potential prognostic marker for older adults with aspiration pneumonia and may act as a proxy for disease severity. Our results support the use of GNRI in the clinical management of aspiration pneumonia.