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. We investigated the clinical characteristics and prognostic significance of GNRI for aspiration pneumonia in older adult patients. In this retrospective observational cohort study, conducted in a single-institute acute-phase community hospital, patients with aspiration pneumonia diagnosed at our institute between April 2014 and March 2016 were enrolled. Data on patient characteristics, microbiological findings, and clinical course were collected. The 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. Overall, 587 aspiration pneumonia patients aged ≥ 65years were enrolled. Their mean age was 86years. Among them, 97 (16.5%) died. In ROC analysis for in-hospital mortality, as compared to albumin, body mass index, and A-DROP score, GNRI had a greater area under the curve value, with a significant difference between GNRI and albumin (p = 0.0058). Male sex (p = 0.028), chronic heart failure (p = 0.023), history of malignancy (p = 0.0025), lower GNRI (p < 0.001), and initial antibiotic change (p < 0.001) were identified as independent adverse prognostic factors in multivariate analysis. 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.