Aim: To investigate the predictive capacity of a systemic immune-inflammation index (SII) in detecting new onset atrial fibrillation (NOAF) following ST segment elevation myocardial infarction (STEMI). Patients & methods: A total of 402 STEMI patients were enrolled in the study. The patients were divided into two groups according to NOAF development. Results: A cut-off point of 1,228,000 for SII was identified with 60% sensitivity and 78.1% specificity to predict NOAF following STEMI. According to pairwise analysis of receiver operating characteristic curve analysis, the predictive power of SII in detecting NOAF following STEMI was similar to high-sensitive C-reactive protein, and better than neutrophil-to-lymphocyte ratio or platelet-to-lymphocyte ratio. Conclusion: SII can be used as one of the independent predictors of NOAF following STEMI.