PURPOSE: This study aimed to investigate the risk factors for severe radiation pneumonitis (RP) after thoracic radiotherapy (RT) in patients with locally advanced non-small cell lung cancer (NSCLC), develop a prediction model to identify high-risk groups, and investigate the impact of severe RP on overall survival (OS). METHODS: We retrospectively collected clinical, dosimetric, and hematological factors of patients with stage III NSCLC receiving thoracic RT without immunotherapy. The primary and secondary end points were severe RP and OS, respectively. Fine-Gray competing risk regression analyses were used to identify the risk factors for severe RP. The patients were randomly divided into training and validation cohorts at a ratio of 6:4. The model was evaluated using receiver operating characteristic (ROC) and calibration curves, and decision curve analysis (DCA). The OS of patients in the RP vs. non-RP and mild RP vs. severe RP groups was analyzed using the Kaplan-Meier method. RESULTS: A total of 305 patients were enrolled in the analysis, and 32 (10.5%) developed severe RP. Interstitial lung disease (ILD) (P = .013), percentage of ipsilateral lung volume receiving ≥ 20 Gy (ipsilateral V CONCLUSION: ILD, ipsilateral V