Introduction Readmissions after stroke are costly. Risk assessment using information available upon admission could identify high-risk patients for potential interventions to reduce readmissions. Baseline stroke severity has been suspected to be a factor in readmission; however, the exact nature of the impact has not been adequately understood. Methods Hospitalized adult patients with first-ever ischemic stroke were identified from a nationwide administrative database. Stroke severity was assessed using a validated claims-based stroke severity index. Cox proportional hazards models were used to investigate the relationship between stroke severity and first readmission within one year. Results Of the 10,877 patients, 4295 (39.5%) were readmitted in one year. The cumulative risk of readmission was 34.1%, 44.7%, and 62.9% in patients with mild, moderate, and severe stroke, respectively. Patients with greater stroke severity had a significantly higher adjusted risk of first readmission for infection, metabolic disorders, neurological sequelae, and pulmonary diseases, whereas those with lesser stroke severity were prone to first readmission due to accidents. Stroke severity did not affect the risk of first readmission for recurrent stroke/transient ischemic attack, other cardiovascular events, malignancy, ulcers/upper gastrointestinal bleeding, kidney diseases, and others. Conclusions Stroke severity in patients with first-ever ischemic stroke not only predicts readmission but also relates to the cause of readmission. Our results might provide important information for tailoring discharge planning to prevent readmissions.