Older patients have a higher risk of aspiration pneumonia and mortality if they are hospitalized. We aimed to assess the effectiveness of an aspiration prevention quality improvement (QI) program that utilizes the Gugging Swallowing Screen (GUSS) in older patients. This retrospective cohort study was conducted in an acute medical care unit of a tertiary hospital in South Korea. The study used one-to-one propensity matching and included 96 patients who received the QI program and 96 who did not. All patients were aged 65 years or older and had risk factors for aspiration, including neurological and non-neurological disorders, neuromuscular disorders, impaired airway defenses, and dysphagia due to esophageal or gastrointestinal disorders. The primary outcomes included the duration of the fasting period during hospitalization, changes in nutritional status before admission and at discharge, in-hospital mortality, and readmission due to pneumonia within 90 days. Fasting period, changes in weight and albumin levels upon discharge after hospitalization, and length of stay did not differ significantly between patients in the GUSS and non-GUSS groups. However, the risk of readmission within 90 days was significantly lower in patients who underwent the GUSS than in those who did not (hazard ratio, 0.085; 95% confidence interval, 0.025-0.290; p = 0.001). The GUSS aspiration prevention program effectively prevented readmission due to pneumonia within 90 days in older patients with acute illnesses. This implies that the adoption of efficient aspiration prevention methods in older patients with acute illnesses could play a pivotal role by enhancing patient outcomes and potentially mitigating the healthcare costs linked to readmissions.