BACKGROUND New-onset atrial fibrillation (NOAF) frequently emerges as a complication in patients with non-ST-segment elevation myocardial infarction (NSTEMI). The relationship between the C₂HEST score, a risk stratification tool for cardiovascular diseases, and NOAF in patients with NSTEMI remains unclear. The objective of this study was to investigate the relationship between the C₂HEST score and NOAF in patients with NSTEMI. MATERIAL AND METHODS Patients who received a diagnosis of NSTEMI from September 2022 to March 2025 were consecutively enrolled. All patients successfully underwent percutaneous coronary intervention within 24 h (thrombolysis in myocardial infarction grade 3), and received electrocardiogram monitoring for at least 36 h. The C₂HEST score was calculated based on coronary artery disease, chronic obstructive pulmonary disease, hypertension, age, heart failure, and hyperthyroidism. RESULTS A total of 665 patients were enrolled in this study. The average age was 63.31±13.09 years, and male patients accounted for 73.08%. During hospitalization, 63 patients (9.47%) developed NOAF. Multivariate logistic regression analysis indicated that C₂HEST score (OR=1.95, 95% CI: 1.63-2.33) was an independent risk predictor for NOAF. Restricted cubic spline analysis revealed a linear correlation between the C₂HEST score and NOAF (P for overall <0.001). Receiver operating characteristic curve showed that the area under the curve of the C₂HEST score was 0.775 (95% CI: 0.724-0.836); the cut-off value was 2.5. CONCLUSIONS Elevated C₂HEST score was found to be an independent risk factor for the development of NOAF in patients with NSTEMI. There was a linear dose-response relationship between C₂HEST score and NOAF.